Document:

<PAGE>   1
                                                                    Exhibit 4.2

                             PARTICIPATION AGREEMENT

         PARTICIPATION AGREEMENT dated as of March 6, 2000 (the "CLOSING DATE"),
by and among MMXI Europe B.V., a private company with limited liability
organized under the laws of the Netherlands (the "COMPANY"), Osprey Research
B.V. ("OSPREY"), a private company with limited liability organized under the
laws of the Netherlands and wholly-owned subsidiary of SIFO Group AB ("SIFO"), a
Swedish corporation and SIFO.

                                  WITNESSETH:

         WHEREAS, the Company wishes to issue to Osprey, and Osprey wishes to
receive from the Company, an equity interest in the Company.

         NOW, THEREFORE, in consideration of the mutual covenants and agreements
herein contained, the companies hereby covenant and agree as follows:

         1.       INITIAL CONTRIBUTION.

         On the date hereof, Osprey shall contribute to the Company
(euro)173,913 in cash and enter into the Agreements (as defined below) in
exchange for 1,739 shares of the Company's share capital (the "OSPREY SHARES").
As of the date hereof, the Osprey Shares represent a 7.99% equity interest in
the Company.

         2.       ADDITIONAL CONTRIBUTIONS.

         Osprey shall make additional contributions to the Company, from time to
time, as set forth in the Shareholders' Agreement (as defined below).

         3.       AGREEMENTS.

         On the date hereof, Osprey shall enter into:

         (a) the Amendment to Shareholders' Agreement, dated the date hereof,
among the Company, Media Metrix, Inc. ("MMXI"), GfK AG ("GFK") and IPSOS SA
("IPSOS") (the "SHAREHOLDERS' AGREEMENT AMENDMENT"), and attached hereto as
Exhibit A;

         (b) the Media Metrix, Inc. Stock Option Agreement, dated the date
hereof, between MMXI and Osprey, and attached hereto as Exhibit B;

         (c) the Amendment to Registration Rights Agreement, dated the date
hereof, among MMXI, GfK, IPSOS and Osprey, and attached hereto as Exhibit C;

         (d) the Deed of Issue for shares of the Company, dated the date hereof,
and attached
<PAGE>   2
 hereto as Exhibit D; and

         (e) this Agreement (such documents are collectively referred to herein
as the "AGREEMENTS").

         4.         REPRESENTATIONS AND WARRANTIES BY THE COMPANY.

         The Company represents and warrants to Osprey as follows:

         (a) The Company is a corporation duly organized and validly existing
under the laws of The Netherlands.

         (b) The Company has the full right, power and authority to enter into
the Agreements to which it is a party (the "COMPANY AGREEMENTS") and to perform
its obligations thereunder. The Company Agreements have been duly authorized,
executed and delivered by it, and the Company Agreements constitute its valid
and binding obligations, enforceable in accordance with their terms, except as
enforcement may be limited by bankruptcy, insolvency, moratorium or other laws
affecting creditors' rights generally, or equity principles, whether in a
proceeding in equity or law.

         (c) The Company is not a party to any contract or other arrangement of
any nature that will materially interfere with its full, due and complete
performance of the Company Agreements to which the Company is a party.

         (d) There is no litigation or proceeding pending nor, to the best of
the Company's knowledge and belief, is any investigation pending or litigation,
proceeding, or investigation threatened involving the Company or any of its
affiliates which could, if adversely determined, materially and adversely affect
the operation or financial condition of the Company or the performance of the
Company's obligations under the Company Agreements.

         (e) The execution, delivery and performance of the Company Agreements
by the Company will not result in any violation of, be in conflict with, or
constitute a default under, with or without the passage of time or the giving of
notice: (i) any provision of the articles of association or memorandum of
association of the Company, (ii) any material contract or other arrangement to
which the Company is a party or by which the Company is bound, or (iii) any
statute, rule or governmental regulation applicable to the Company.

         5.       REPRESENTATIONS AND WARRANTIES BY OSPREY.

         Osprey represents and warrants to the Company as follows:

         (a) Osprey is a corporation duly organized and validly existing under
the laws of The Netherlands.
<PAGE>   3
         (b) Osprey has the full right, power and authority to enter into the
Agreements and to perform its obligations thereunder. The Agreements have been
duly authorized, executed and delivered by it, and the Agreements constitute its
valid and binding obligations, enforceable in accordance with their terms,
except as enforcement may be limited by bankruptcy, insolvency, moratorium or
other laws affecting creditors' rights generally, or equitable principles,
whether in a proceeding in equity or law.

         (c) Osprey is not a party to any contract or other arrangement of any
nature that will materially interfere with its full, due and complete
performance of the Agreements.

         (d) There is no litigation or proceeding pending nor, to the best of
Osprey's knowledge and belief, is any investigation pending or litigation,
proceeding, or investigation threatened involving Osprey or any of its
affiliates which could, if adversely determined, materially and adversely affect
the operation or financial condition of the Company or the performance of
Osprey's obligations under the Agreements.

         (e) The execution, delivery and performance of the Agreements by Osprey
will not result in any violation of, be in conflict with, or constitute a
default under, with or without the passage of time or the giving of notice: (i)
any provision of the organizational documents of Osprey, (ii) any material
contract or other arrangement to which Osprey is a party or by which Osprey is
bound, or (iii) any statute, rule or governmental regulation applicable to
Osprey.

         6.       CONDITIONS TO CLOSING.

         On or prior to the date hereof, the following conditions shall have
been met:

         (a) Osprey shall have made the initial contribution and shall have
executed and delivered the Agreements to which it is a party.

         (b) The Company shall have executed and delivered the Amendment to
Shareholders' Agreement, dated as of the date hereof, between the Company and
MMXI.

         (c) Each party to the Agreements other than Osprey and the Company
shall have executed and delivered the Agreement or Agreements to which it is
party.

         7.       SIFO GUARANTEE.

         SIFO hereby unconditionally and irrevocably guarantees the obligations
of Osprey under the Agreements.

         8.       GOVERNING LAW.

         The validity, performance, and all matters relating to the
interpretation and effect of this Agreement shall be governed by the laws of The
Netherlands, without regard to principles of law (such as "conflicts of law")
that might make the law of some other jurisdiction applicable.
<PAGE>   4
         9.       COUNTERPARTS.

         This Agreement may be executed in counterparts, each of which shall be
deemed an original and each of which shall constitute one and the same
Agreement.

         10.      ARBITRATION.

         All disputes arising out of this Agreement, including, without
limitation, disputes concerning the existence and validity thereof, shall be
resolved by binding arbitration in accordance with the rules of the Netherlands
Arbitration Institute (Netherlands Arbitrage Instituut, the "NAI") and in
accordance with the following:

                  (a) the arbitral tribunal shall be comprised of three
         arbitrators, one of whom shall be appointed by the Company, one of whom
         shall be appointed by the Osprey and one of whom shall be appointed by
         the first two arbitrators;

                  (b)      the place of arbitration shall be Amsterdam, The
         Netherlands;

                  (c)      all proceedings shall be conducted in the English
         language;

                  (d)      the tribunal shall make its determination in
         accordance with the rules of the law;

                  (e) the proceedings may not be consolidated with any other
         arbitration proceedings pursuant to Section 1046 of the Code of the
         Civil Procedure (Wetboek van urgerlijke Rechtsvordering), unless such
         other proceedings are conducted exclusively between the parties to this
         Option; and

                  (f)      the arbitral judgment may not be published by the
         NAI.
<PAGE>   5
         IN WITNESS WHEREOF, the parties hereto have executed this Agreement as
of the day and year first above written.

                                    MMXI EUROPE B.V.

                                    By: /s/ Arielle Dinard
                                    ----------------------
                                    Name:   Arielle Dinard
                                    Title:   Managing Director

                                    OSPREY Research B.V.

                                    By:  /s/ Jan-Erik Jannsson
                                    --------------------------
                                    Name:    Jan-Erik Jannsson
                                    Title:   Director

                                    SIFO GROUP AB (publ)

                                    By: /s/ Jan-Erik Jannsson
                                    -------------------------
                                    Name:    Jan-Erik Jannsson
                                    Title:   Chief Financial Officer
                                             (power of attorney)
<PAGE>   6
                              SCHEDULE OF EXHIBITS

Exhibit A -       Amendment to Shareholders' Agreement

Exhibit B -       Media Metrix, Inc. Stock Option Agreement

Exhibit C -       Amendment to Registration Rights Agreement

Exhibit D-        Deed of Issue<PAGE>   1
                                                                     Exhibit 4.3

                      AMENDMENT TO SHAREHOLDERS' AGREEMENT

         AMENDMENT, dated as of March 6, 2000, to Shareholders' Agreement dated
as of September 1, 1999 (the "SHAREHOLDERS' AGREEMENT"), by and among MMXI
Europe B.V., a private company with limited liability organized under the laws
of the Netherlands (the "COMPANY"), Media Metrix, Inc., a Delaware corporation,
("MMXI"), GfK AG, a German stock corporation ("GFK"), IPSOS SA, a French stock
corporation ("IPSOS"), and Osprey Research B.V. ("OSPREY"), a private company
with limited liability organized under the laws of the Netherlands and
wholly-owned subsidiary of SIFO Group AB, a Swedish corporation.

         WHEREAS, MMXI, GfK and IPSOS are parties to the Shareholders' Agreement
and constitute the holders of all of the Company's share capital; and

         WHEREAS, the Company is issuing additional share capital to Osprey as
of the date hereof; and

         WHEREAS, the addition of Osprey as a party to the Shareholders'
Agreement is a condition to the issuance of such shares to Osprey.

         NOW, THEREFORE, in consideration of the premises and mutual covenants
contained herein, the parties hereto agree as follows:

         1.       CONTINUATION OF SHAREHOLDERS' AGREEMENT

         The Shareholders' Agreement shall continue in full force and effect as
amended by this Amendment. All capitalized terms not otherwise defined in this
Amendment shall have the meanings given in the Shareholders' Agreement.

         2.       ADDITION OF OSPREY AS A SHAREHOLDER

         The Shareholders' Agreement is hereby amended to add Osprey as a
Shareholder for all purposes thereunder.

         3.       COMPOSITION OF SUPERVISORY BOARD

         The first three sentences of Section 3(a) of the Shareholders'
Agreement are hereby amended and restated to read as follows:

                  The Supervisory Board of the Company (the "SUPERVISORY BOARD")
                  shall be comprised of six members (each, a "SUPERVISORY
                  DIRECTOR"), each of whom shall have an equal vote on matters
                  brought before the
<PAGE>   2
                  Supervisory Board. The Shareholders agree that (i) MMXI shall
                  have the right to nominate three (3) Supervisory Directors;
                  (ii) GfK shall have the right to nominate one (1) Supervisory
                  Director; (iii) IPSOS shall have the right to nominate one (1)
                  Supervisory Director; and (iv) Osprey shall have the right to
                  nominate one (1) Supervisory Director.

         4.       MEETINGS AND RESPONSIBILITIES OF THE SUPERVISORY BOARD

         Section 4(b)(xi) of the Shareholders' Agreement is hereby amended and
restated to read as follows:

                  Entering into any partnership, joint venture or similar
                  arrangement involving equity ownership with the competitors of
                  GfK, IPSOS or Osprey listed on Schedule B hereto, it being
                  specified that the contents of Schedule B may be modified by
                  GfK, IPSOS or Osprey from time to time;

         5.       SUPERVISORY BOARD VOTING

         (a) The first sentence of Section 5(a) of the Shareholders' Agreement
is hereby amended and restated to read as follows:

                  Except as set forth in subsections (b), (c) and (d) below,
                  actions of the Supervisory Board shall be approved at a
                  meeting of the Supervisory Board by a majority per capita vote
                  so long as there is a quorum.

         (b) Section 5(b) of the Shareholders' Agreement is hereby amended and
restated to read as follows:

                  In addition to any votes of the general meeting required by
                  applicable law, approval of matters listed in Sections
                  4(b)(ix)-(xxi) shall require the unanimous consent of all of
                  the Supervisory Directors then in office, whether present or
                  not at a meeting of the Supervisory Board; provided, however,
                  that Osprey shall cause the Supervisory Director it nominates
                  to the Supervisory Board to vote in favor of any matter listed
                  in Sections 4(b)(x)-(xxi) that is supported by all other
                  Supervisory Directors.

         (c)      The Shareholders' Agreement is hereby amended to add a new
Section5(d), to read as follows:

                  (d) The determination of whether to exercise the option to
                  purchase shares of RelevantKnowledge Holding AB from MMXI
                  pursuant to the terms of the Option to Purchase Shares, dated
                  as of

                                      -2-
<PAGE>   3
                  March 6, 2000, shall be made by the unanimous vote of the
                  Supervisory Directors nominated by GfK and IPSOS. MMXI shall
                  cause the Supervisory Directors nominated by MMXI to abstain
                  from such vote.

         6.       DEADLOCKS

         Section 6(b) of the Shareholders Agreement is hereby amended and
restated to read as follows:

                  Should the matter not be resolved by the required majority or
                  supermajority vote at the Second Board meeting, GfK, IPSOS
                  and/or Osprey (the "DISAGREEING PARTY"), as the case may be,
                  shall have the right (such right, a "DISAGREEING PARTY
                  RIGHT"):

                           (i) to offer the Shares owned by such Disagreeing
                  Party to the other Shareholders or to the Company at a price
                  determined in accordance with Paragraph 6(e) below; or

                           (ii) to exercise such Disagreeing Party's right to
                  exchange its Shares for shares of MMXI common stock in
                  accordance with the Option Agreement (the "OPTION AGREEMENT")
                  between MMXI and such Shareholder; or

                           (iii) if neither the other Shareholders nor the
                  Company are willing (and permitted in accordance with
                  applicable law) to purchase the Shares, to offer its Shares to
                  a third party in accordance with the provisions of Paragraph
                  11(e) of this Agreement.

         7.       SHAREHOLDER VOTING

         Each Shareholder shall vote all of its Shares as necessary to make
changes to the Company's Articles of Association to conform the Articles of
Association to the terms and provisions of this Amendment to the extent
permitted under the law of the Netherlands.

         8.       MANAGING BOARD

         The Shareholders agree to vote their Shares to elect Lars Bjorkman as
an additional member of the Managing Board promptly following the date hereof to
serve as provided in the Agreement and the Articles of Association.

         9.       CAPITAL CONTRIBUTIONS IN THE COMPANY

                                      -3-
<PAGE>   4
         (a) On the date hereof, Osprey has contributed cash to the Company in
the amount of (euro) 173,913 and has been issued 1,739 Shares.

         (b) Section 10(b)(v) of the Shareholders' Agreement is hereby amended
to increase the maximum aggregate amount of Additional Contributions from
(euro) 7,000,000 to (euro) 7,782,609 and to increase the Maximum Commitment from
(euro) 9,000,000 to (euro) 9,782,609.

         (c) The first sentence of Section 10(c) of the Shareholders' Agreement
is hereby amended and restated to read as follows:

                  If the Shareholders elect to make an Additional Contribution
                  in excess of the Maximum Commitment, MMXI shall grant to each
                  of IPSOS, GfK and Osprey an option to exchange the Shares or
                  evidence of indebtedness issued in respect of such Additional
                  Contribution for the number of shares of common stock, par
                  value $.01 per share, of MMXI ("MMXI COMMON STOCK") determined
                  by dividing (i) the amount of the Additional Contribution made
                  by such Shareholder (expressed in U.S. Dollars at the rate at
                  which Euros are converted into U.S. Dollars as posted in The
                  Wall Street Journal the business day prior to the Additional
                  Contribution) by (ii) $59.96, in the case of GfK and IPSOS,
                  and 100% of the closing price of the MMXI Common Stock on the
                  Nasdaq National Market on the date hereof, in the case of
                  Osprey.

         10.      CHANGE IN CONTROL

         (a) Section 12(a) of the Shareholders' Agreement is hereby amended and
restated to read as follows:

                  Upon a Change in Control at GfK, IPSOS, Osprey or SIFO Group
                  AB (the "DEPARTING SHAREHOLDER"), the remaining Shareholders
                  (other than MMXI) shall have the right to purchase such Shares
                  at a purchase price determined in the manner provided for in
                  Paragraph 6(e) hereof (except that in the event of a Change in
                  Control of SIFO Group AB, Osprey shall have no right to
                  purchase such Shares). The Departing Shareholder shall notify
                  such remaining Shareholders and the Company promptly of a
                  Change of Control or a pending Change of Control. If the
                  Departing Shareholder and such remaining Shareholders are not
                  able to agree upon a price for the Shares within ten (10)
                  days, an independent auditor shall determine the fair market
                  value of the Shares in accordance with Paragraph 6(e), and
                  each such remaining Shareholder shall have ten (10) days from
                  the date of the determination of the price to notify the
                  Departing Shareholder of its

                                      -4-
<PAGE>   5
                  intention to purchase such Shares (or if both such remaining
                  Shareholders desire to purchase such Shares, its intention to
                  purchase its pro rata portion of such Shares) in accordance
                  with the procedures established under Paragraph 11 hereof. If
                  such remaining Shareholders do not exercise such right, the
                  Company shall have an opportunity to repurchase, to the extent
                  permitted under the law of the Netherlands, the Departing
                  Shareholder's shares in accordance with the procedures
                  established under Paragraph 11 hereof; provided, however, that
                  neither such remaining Shareholders nor the Company will have
                  a right to purchase or repurchase the Shares if the Departing
                  Shareholder exercises its right under the Option Agreement to
                  exchange its Shares for MMXI Common Stock.

         (b) Section 12(b) of the Shareholders' Agreement is hereby amended and
restated to read as follows:

                  Upon a Change of Control at MMXI during the term of the Option
                  Agreements between MMXI and each of GfK, IPSOS and Osprey,
                  GfK, IPSOS and Osprey shall have the right to exchange all of
                  their respective Shares for shares of MMXI Common Stock in
                  accordance with the Option Agreements. Upon a Change in
                  Control at MMXI following the expiration of the Option
                  Agreements, each of GfK, IPSOS and Osprey shall have the
                  option to sell all of their respective Shares, and MMXI hereby
                  agrees to purchase such Shares, for a purchase price
                  determined in the manner provided for in Paragraph 6(e)
                  hereof.

         11.      TERMINATION OF CERTAIN PROVISIONS

         Upon the earlier of (a) the exercise of the Option granted by IPSOS to
GfK pursuant to Section 13 of the Shareholders' Agreement and (b) June 4, 2000,
Section 13 of the Shareholders' Agreement shall be deleted and shall have no
further effect.

         12.      NON-COMPETITION; NON-SOLICITATION

         The final sentence of Section 17(a) of the Shareholders' Agreement is
hereby amended and restated to read as follows:

                  The Company agrees that neither it nor any of its officers,
                  directors or subsidiaries will, directly or indirectly engage
                  in the business of audience measurement in television, radio,
                  newspaper and magazines for so long as GfK, IPSOS or Osprey
                  are parties to this Agreement

                                      -5-
<PAGE>   6
                  and for two years thereafter.

         13.      ARBITRATION

         Section 22(c)(i) of the Shareholders' Agreement is hereby amended and
restated to read as follows:

                  The arbitral tribunal shall be comprised of three arbitrators,
                  one of whom shall be appointed by MMXI, one of whom shall be
                  appointed by GfK, IPSOS and Osprey voting in proportion to
                  their respective shareholdings, and one of whom shall be
                  appointed by the other two arbitrators;

         14.      TRANSFER OF SHARES

         Notwithstanding Section 11 of the Shareholders' Agreement, Osprey shall
have the right to sell its Shares to SIFO Group AB so long as SIFO Group AB has
agreed to become a party to, and be bound by, the Shareholders' Agreement. Upon
such an event, all of Osprey's rights and obligations under the Shareholders'
Agreement shall be assigned to SIFO Group AB. Upon such transfer, MMXI, GfK and
IPSOS each agree not to exercise any preemptive right set forth in the Company's
Articles of Association.

         15.      MISCELLANEOUS

         (a) Any notices, consents or other communication required to be sent or
given to Osprey hereunder or under the Shareholders' Agreement by any of the
parties shall in every case be delivered at the addresses as set forth below or
at such other addresses as may be furnished in writing:

                           Osprey Research B.V.
                           c/o SIFO Group AB
                           World Trade Center
                           Box 70408, SE-107
                           25 Stockholm, Sweden

                           Attention: Mr. Robert Kessiakoff
                           Facsimile: +46 8 698 0933

                           With a copy to:

                                      -6-
<PAGE>   7
                           Peter Torngren
                           Box 1703, S-111
                           87 Stockholm, Sweden

                           Facsimile: +46 8 614 3190

         (b) This Amendment shall be governed exclusively by and construed and
enforced in accordance with the internal laws of The Netherlands. This Amendment
shall be binding upon and inure to the benefit of the parties signatory hereto
and their respective successors and assigns.

         (c) This Amendment may be executed in a number of counterparts, all of
which together shall for all purposes constitute one agreement, binding on all
the parties hereto notwithstanding that all such parties have not signed the
same counterpart.

                                      -7-
<PAGE>   8
         IN WITNESS WHEREOF, the parties hereto have executed this Amendment as
of the date and year first above written.

                     MMXI EUROPE B.V.

                     By: /s/ Arielle Dinard
                         __________________________________________
                         Name: Arielle Dinard
                         Title: Managing Director

                     MEDIA METRIX, INC.

                     By: /s/ Thomas A. Lynch
                         __________________________________________
                         Name: Thomas A. Lynch
                         Title: Chief Financial Officer

                     GFK AG

                     By: /s/ Gerhard Kirschner
                         __________________________________________
                         Name: Gerhard Kirschner
                         Title: Member of the Board

                     IPSOS SA

                     By: /s/ Didier Truchot
                         __________________________________________
                         Name: Didier Truchot
                         Title: Chairman and Chief Executive Officer

                     OSPREY  RESEARCH B.V.

                     By: /s/ Jan-Erik Jannsson
                         __________________________________________
                         Name: Jan-Erik Jannsson
                         Title: Director
<PAGE>   9
                                                                      SCHEDULE B

                          COMPETITORS OF SIFO GROUP AB

@Plan
A.C. Nielsen
AEGIS
AGB Italia Group
Arbitron
ENIGMA
Fletcher Research
Forrester Research
Gallup
Gartner Group
Icon
INRA
IntelliQuest Inc.
International Data Corporation
Jupiter Communications
Mediametrie
NetRatings
NetValue
NFO Infratest Burke Group
Nielsen Media Research
PC Data Primedia
Information Inc.
Taylor Nelson SOFRES
The Gallup Organization
United Information Group
VNU
Westat
WPP Group

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