Q:

Is Facebook allowed to sell/give your information away to anyone willing to buy it? 

Is Facebook allowed to sell information about their users to other companies? 
For example selling name, address and IP information on a specific geographic location could be very valuable information for competitive ISP's trying to win customers from each other. 
According to Facebook's privacy help page it sais:

While you are allowing us to use the information we receive about you, you always own all of your information.  Your trust is important to us, which is why we don't share information we receive about you with others unless we have:

received your permission;
given you notice, such as by telling you about it in this policy; or
removed your name or any other personally identifying information from it. 

By registering at Facebook, have I already given my permission? 

A:

There is a classic phrase:  

"If you are not paying for it, you're not the customer; you're the
  product being sold"

In case of companies' liquidation, they openly sell their user databases on the internet as one of the liquidated assets.  
Well, think this way.  If Microsoft bought , say, Skype, had Skype sold its user database and what is the sense/value of Skype, or Facebook, without their user database? 
Besides, online service companies, like Facebook, are functioning on the basis of license (terms of service, etc.)  agreements with users not contracts. 
That means that if one of the sides breaks it, this would constitute violation of copyright law not contract law.   
Update:
Here is an excerpt from  

Hundreds of websites share usernames sans permission. 
Photobucket, Wall Street Journal, Home Depot take liberties with your personal info
http://www.theregister.co.uk/2011/10/11/websites_share_usernames/ 

"Home Depot, The Wall Street Journal, Photobucket, and
hundreds of other websites share visitor's names, usernames, or other
personal information with advertisers or other third parties, often
without disclosing the practice in privacy policies, academic
researchers said.    
Sixty-one percent of websites tested by researchers from Stanford Law
School's Center for Internet and Society leaked the personal
information, sometimes to dozens of third-party partners.  Home Depot,
for example, disclosed the first names and email addresses of visitors
who clicked on an ad to 13 companies.  The Wall Street Journal divulged
to seven of its partners the email address of users who enter the
wrong password.  And Photobucket handed over the usernames of those who
use the site to share images with their friends."  

Your phone company is selling your personal data (CNN, Nov 1, 2011)
"Verizon (VZ, Fortune 500) is the first mobile provider to publicly confirm that it is actually selling information gleaned from its customers directly to businesses.  But it's hardly alone in using data about its subscribers to make extra cash"
Facebook is blurting out your private information
no date but comments start on Oct, 2010, and the author regularly tweet this article (Nov, 2011)   
"... the moment you land into one of their [Facebook's] “trusted partners” sites, your personal information has just been given away" 

I could not resist from visualizing the comment by Hendrik Brummermann from this answer here pointing to this image found on the web:
 
as well as to answer that answer by quote of "Privacy Zuckering" definition:

"The act of creating deliberately confusing jargon and user-interfaces which trick your users into sharing more info about themselves than they really want to." 
(As defined by the EFF).    
The term "Zuckering" was suggested in an EFF article by Tim Jones on Facebook's "Evil Interfaces".  It is, of course, named after Facebook CEO Mark Zuckerberg  

A:

Yes - Facebook's customers are not you, me or other individuals, but companies.  Individuals and their data are the product, not the customer. 
Companies who provide a Facebook app can get any information about you that they want. 
Facebook is allowed to provide this information to their customers - you can prevent it by never installing any Facebook apps, and ensuring all information privacy checkboxes are selected, but the defaults let your information go to all manner of companies in many countries! 

A:

Note that the FTC recently filed a suit against Facebook alleging that Facebook had violated their own privacy promises.   The FTC's suit was successful; Facebook settled and agreed to change their practices. 
The FTC accused Facebook of violating their own public promises and privacy policy in multiple ways.   The charges are rather remarkable.   Among them:

"Facebook promised users that it would not share their personal information with advertisers.  It did." 
"In December 2009, Facebook changed its website so certain information that users may have designated as private – such as their Friends List – was made public.  They didn't warn users that this change was coming, or get their approval in advance." 
"Facebook told users they could restrict sharing of data to limited audiences – for example with "Friends Only."  In fact, selecting "Friends Only" did not prevent their information from being shared with third-party applications their friends used." 
"Facebook represented that third-party apps that users' installed would have access only to user information that they needed to operate.  In fact, the apps could access nearly all of users' personal data – data the apps didn't need." 

See the FTC complaint for further details. 
Even today, after the FTC settlement, it is important to understand that any third-party application you install will potentially get access to a lot of information that Facebook has about you, even if you selected privacy options in an attempt to keep that information private.   Those applications could sell that information to others.