Weaving SIOC into the Web of Linked Data
Uldis Bojārs

Alexandre PassantDigital Enterprise ResearchInstitute
National University of Ireland,
GalwayElectricité de France, R&D
Clamart, France& LaLIC, Université
Paris-Sorbonne
Paris, France

uldis.bojars@deri.org
Richard Cyganiak

alexandre.passant@edf.fr
John Breslin

Digital Enterprise ResearchInstitute
National University of Ireland,
GalwayDigital Enterprise ResearchInstitute
National University of Ireland,
Galwayrichard@cyganiak.dejohn.breslin@deri.org

ABSTRACTSocial media sites can act as a rich source of large amountsof data by letting anyone easily create content on the Web.
The SIOC ontology and tools developed for it allow us to express this information as interlinked RDF data. This paper
describes approaches for making this Social Web information
an integral part of the Web of Linked Data.

1.

INTRODUCTION

By envisioning a Web of Data, in addition to the Web ofDocuments, the Semantic Web provides a way to represent
real-world data, as well as virtual objects in a uniform manner. The Social Web can also benefit from a common way
to link and represent content created through online communities and social media websites.
The Web currently uses untyped HTML hyperlinks to connect web pages and mainly considers documents without any
semantics. RDF allows us to describe information about resources, including real-world objects, and the different typesof relations between them. The Linked Data initiative takes
this a step further and defines best practices for publishinglinked data on the web [2]: use URIs as names for things;
use HTTP URIs so that people can look up those names;
when someone looks up a URI, provide useful information;
include links to other URIs so that people can discover more
related things.
SIOC (Semantically Interlinked Online Communities) [7] isa step towards providing linked data from social media sites(forums, blogs, wikis, etc.) and provides a common vocabulary to describe meta-data of such sites in RDF. The project

itself is built in a collaborative way, since it involves many
partners and welcomes any suggestions on its SIOC-DEV
mailing list workgroup1 , and its work has been recently published as a W3C member submission2 .
This paper describes how Social Web data, described in
SIOC, can be further weaved together with other kinds oflinked data. We describe the structure of SIOC data andgive a description of different approaches to interlink SIOC
data with other sources of linked data. Finally, we conclude
the paper with some interesting uses of such data.

2.

SIOC DATAThe SIOC implementations list3 contains around 35 applications for creating and using SIOC data. By installing
relevant SIOC export plugins, online community sites cangenerate linked data and start forming a critical mass of
RDF data about user-created content in the same way as
LiveJournal did for FOAF data. Other tools allow users
to browse SIOC data or to translate existing data, such as
mailing list archives, to SIOC. Moreover, SIOC is also used
for enterprise data integration[9], and some popular Web 2.0
sites such as Seesmic4 start using it to model their data.
To demonstrate the linked nature of SIOC data created bythese sites, let us look at data generated by a SIOC export
plugin for a blog site (e.g. the WordPress SIOC plugin5 ).
It creates a set of RDF documents describing the blog itself
and every post, comment and user on this blog. Note thatdata instances exported from a larger site (e.g., a bulletinboard) are similar, but larger in number and may containmultiple sioc:Forum objects whereas a blog has just one.
An important property of SIOC data is that all the RDF
documents generated by an exporter are interlinked using
rdfs:seeAlso links (Fig. 1):1

http://groups.google.com/group/sioc-dev
http://www.w3.org/Submission/2007/02/
3
http://rdfs.org/sioc/applications/4
http://seesmic.com
5
http://sioc-project.org/wordpress
2

Copyright is held by the author/owner(s).
LDOW2008, April 22, 2008, Beijing, China.

Sitesioc:Site

Author

....

....

....

....

....

....

Posts

Comments

Figure 1: Structure of linked SIOC data
• a blog profile links to all posts on the blog;

Figure 2: Interlinking SIOC, FOAF and SKOS

• posts link to replies / comments;• posts and comments link to the profile of a user who
created them (if a user is registered on a site).

linking to SIOC Sites(s). Yet, there are some things thatcan be done to facilitate linking to SIOC:

This makes SIOC-enabled social media sites a rich source ofinterlinked data and enables an RDF crawler to start at themain profile of a site and retrieve all SIOC RDF pages that
this site provides from a single entry point, i.e. the RDF filedescribing the sioc:Site. Almost all SIOC-enabled sites
also have RDF autodiscovery links in their web pages, making it easier to find RDF data for people and applications
when opening a page. Another interesting use of this autodiscovery feature is implemented in the Semantic Radar
Firefox extension6 . The extension automatically sends a notification to the Ping The Semantic Web service7 whenever
the user accesses an autodiscovery-enabled web page. This
allows people to participate in the distributed discovery of
Semantic Web content [6].

• owners of FOAF profiles can link to social media sites
and their user accounts on these sites;

While these features makes a SIOC-enabled site a good sourceof linked data, at this point it can still be viewed as a“walled garden” and not really as an integral part of a larger
Web of Linked Data. On its own, SIOC data from a single exporter may have a limited connectivity to the “outside
world”. Links to other websites are mainly achieved at the
moment thanks to the extraction of HTML hyperlinks frompost content. These hyperlinks are republished in the post’s
SIOC profile using the sioc:links_to property.

3.

LINKING DATA WITH SIOC

There are two paths to making online community data, described in SIOC, a more integrated part of the Web of Data:
linking into “SIOC space”, and referencing linked data frominside online community sites. We will now describe both ofthese paths, starting with linking to SIOC data.

3.1

Linking to SIOC Data

The question of receiving links from the “outside world”,
strictly speaking, is not something SIOC exporters can directly influence as this depends on other sources of data
6
7

http://sioc-project.org/firefox
http://pingthesemanticweb.com• SIOC exporters can be optimized to make SIOC data
easier to discover;• Semantic Web indexing and lookup services can findand provide access to SIOC data.
A simple and effective way to link to SIOC profiles is topoint to them from personal FOAF profiles. For example,
people often already link to their blogs from FOAF using
the foaf:weblog property. In this case they just need to
add an rdfs:seeAlso link pointing to the weblog’s SIOC
profile, which also mentions the weblog’s homepage URI andthus allows a client to connect the information in the two
documents.
People can also use the foaf:holdsOnlineAccount property
to point to user accounts (sioc:User) that they have registered on online community sites (Fig. 2). As a part of
SIOC data, these user accounts are normally linked to content items created by the user.
If we can make it easier for applications and users browsing
the Web to find SIOC data, they are more likely to use thisdata. RDF autodiscovery links play an important role inhelping one to find Semantic Web data, and almost all SIOC
exporters use them, making it a reliable way to detect thepresence of SIOC data. As a result, all web pages on a SIOCenabled site contain RDF autodiscovery links to relevant
RDF data (e.g., a blog post will point to data about thisparticular post).
Another way of improving discoverability is content negotiation. The idea is that client requests to URIs of resources
such as forum posts are answered with either an HTML oran RDF representation, depending on the client’s preferences. This lets existing URIs of the forum’s web pages,which are already used as targets of HTML links, double asaccess points to the SIOC data when used by RDF-awareclients. We have experimented with adding content negotiation to some SIOC exporters, but this has proven to bechallenging. First, implementing full content negotiation as
described in the HTTP specification is fairly complex. Second, it is hard to verify that an implementation works correctly with the large number of different versions of Web
and data browsers in use on the Web, some of which express
questionable preferences. Third, the plugin systems of somecontent management systems do not provide the necessary
hooks for implementing content negotiation. Therefore, we
recommend that linked data browsers such as Tabulator [3]also make use of RDF autodiscovery information as an alternative to content negotiation.
SIOC exporters can also use RDFa8 to embed SIOC data
directly in HTML documents, e.g. blog posts. A first approach to embed RDFa in Drupal has been explored9 , andfuture work may go in this direction, since it provides another way to discover RDF triples related to a document.
Finally, some site administrators have decorated their pages
with small SIOC icons that link directly to the related SIOC
profile. These icons can be considered redundant as we already have RDF autodiscovery links, or can even be considered harmful as users who click the icon in non-RDF-aware
Web browsers will be subjected to raw RDF/XML code. On
the other hand, such icons are a useful marketing tool that
increases awareness of SIOC in general.
In general, it can be said that we desire a user experience
where URIs of Web resources such as blog posts can simply
be used in HTML hyperlinks and RDF statements to referto the resource. Mechanisms such as RDF autodiscovery,
content negotiation or RDFa parsing should be transparent
to the user and entirely reliable.
An important aspect of linked data is being able to findwhat data is linking into a resource. Online community
sites sometimes notify each other of links they make by using pingbacks or trackbacks, but other publishers usually do
not send such notifications. This is where Semantic Web indexing services such as Sindice [12] play an important role.
Such a service can find the incoming RDF statements thatreference a resource. These “backlinks” can be displayed tousers, either on SIOC-enabled sites or directly in the userinterface of linked data browsers, thus helping users to navigate these links in both directions.

3.2

Linking From SIOC Data to the Outside

Some resources that users may want to link to include:• Persons - authors of posts and comments => link totheir FOAF URI and FOAF profile;• Topics - categories and tags => link to other linked
data about these topics;8

http://www.w3.org/TR/xhtml-rdfa-primer/9
http://groups.google.com/group/sioc-dev/browse_
thread/thread/b1585e9ef3a17665• Other data - information associated with / embedded
within content.

FOAF is one of the most successful Semantic Web vocabularies and is often used to describe personal information andsocial network relations. One of the first use cases for linking
to other RDF data is to link posts or comments to FOAF
profiles of their creators. Currently sioc:Post(s) are linked
to a creator’s user profile on a community site, but export
tools can be easily extended to point to a provided FOAF
profile and to the URI of a creator of this content.
In order to point to FOAF profiles, the application has toknow what URI to point to. Users of a community site
(e.g., post authors) can supply this information when registering on the site, but comment authors may not be motivated enough to provide the necessary FOAF information
and an automated process for finding this data is preferable. This can be achieved automatically using OpenID10 :A user authenticates using OpenID and a blog engine checks
an OpenID URI for links to person’s FOAF profile. If sucha link is found, statements pointing to the person’s FOAF
profile (e.g., owl:sameAs and rdfs:seeAlso) can be added
to SIOC metadata describing the author of the comment.
Some work regarding this point are currently done within
the SparqlPress11 project, which aims to provide Semantic
Web functionalities, in both exporting, crawling and linking
RDF data.
Categories and tags which describe the topic of the content
are also good candidates for pointing to additional information. SIOC exporters use a sioc:topic property to point toa topic URI. Topic category hierarchies can be described in
SKOS and topics can be linked to more information about
them such as RDF data from DBpedia [1]. Tag vocabularies
such as the Tag Ontology, MOAT (Meaning Of A Tag)12 , or
SCOT [8] can be used to associate more information withtags. However, SIOC exporters need to know what URIs topoint to for a given tag or category.
One option is to add this knowledge to the site in advance.
Adding annotations every time a new topic is created may
work for categories that do not change very often, but is not
feasible for tags which are more dynamic. Another option isto rely on the MOAT framework, which allows us to assign
meaning to tags in a collaborative way using existing URIs
from datasets such as DBpedia or GeoNames [11]. For example, some users in a community may have indicated that the
sparql tag means http://dbpedia.org/resource/SPARQL.
When a user tags a post with the sparql tag, the DBpedia URI will be offered as a possible meaning of the tag,and can be selected with a single click for exporting as asioc:topic link. This provides an efficient way to interlink
community site posts and DBpedia. Thanks to semantic relationships within DBpedia itself, useful information about
how the topic of different posts relate is only a link away
(Fig. 3).
10

http://apassant.net/blog/2007/09/23/
retrieving-foaf-profile-from-openid/11
http://wiki.foaf-project.org/SparqlPress
12
http://moat-project.org/

http://tags.moatproject.org/tag/sparql

tags:associatedTag

tags:namesparqlhttp://example.org/
tagging/1

http://dbpedia.org/
resource/SPARQL

moat:tagMeaning
sioc:topic

tags:taggedBy

http://example.org/alexskos:subject

tags:taggedResource

http://dbpedia.org/resource/
Category:Web_services

http://example.org/
post/1

sioc:topic

sioc:topic

sioc:has_creator

sioc:has_creator

sioc:has_creator
http://myblog.net/post

http://something.net/post

http://myblog.net/john

http://something.net/uldis

Figure 3: Interlinking blog posts thanks to SIOC,
MOAT and DBpedia.

There are other resources that authors may want to describeor link to. Some SIOC exporters have built-in functionality for defining links to additional RDF data. The Drupal
and WordPress exporters pass post content through a filter
to find hyperlinks with MIME type application/rdf+xml.
Appropriate rdfs:seeAlso statements are added to the generated SIOC RDF data for this post in addition to the usual
sioc:links_to property. This simple mechanism allows us
to add links to RDF data to existing blog posts with very
little effort. As a possible extension of this mechanism, some
information about the linked resources may be copied into
the post’s SIOC profile, e.g. rdfs:label and rdf:type information which assists users of data browsers in navigation.
Sometimes, having a link to external RDF data is not enough.
Within a post, a user may want to express some additional
machine-readable information about the objects discussed in
the post. As example use case an author describes a software
project, points to DOAP (Description Of A Project) data
describing this project, and includes a review of this project.
This information can be added to SIOC data, but we needa way to add additional information to content items and to
be able to retrieve it later.
Such embedded annotations are not widespread yet and current SIOC tools do not implement this functionality, butthey can be extended to support such a use case. Filters for
extracting metadata from post content can be executed one
after another and RDF data can be extracted and added to
the generated SIOC data. Data can be embedded in content
in a number of different ways – RDFa, GRDDL13 , microformats – as long as appropriate content extraction modules
are available.

4.

USING SOCIAL MEDIA DATAThe Social Web is not limited to forums or blogs. There aredifferent kinds of social media and Web 2.0 sites, such as
Flickr, Twitter and Facebook, which offer interesting content that can be described in RDF using FOAF, SIOC and13

http://www.w3.org/TR/grddl-primer/

Figure 4: FOAF, SIOC and Data Portability.

related vocabularies [4]. Developers have already created exporters for such tools and some of them use SIOC to describe
user account and content, as in exporters for Twitter14 andFlickr[10].
Information from users’ contribution to these sites can beinteresting at different levels. There are social networks,
which are formed by users on different sites. These networks can be described in FOAF. Web users may also havea FOAF profile which points to their different accounts on
different sites. Users create content items (sioc:Item) suchas videos, bookmarks, etc. which can be organised in containers (Fig. 4). When expressed in RDF, this information forms an interlinked web of rich social data, ready forreuse. Moreover, the use of the SIOC types module15 allow people to decribe exactly the type of their content (eg:sioct:BlogPost, sioct:VideoChannel).
Two interesting and emerging applications of such information are object-centred sociality and social media portability. Object-centred sociality looks at objects such as content
items which people create and co-annotate as a medium
through which people are connected together. The use of
SIOC data for object-centred sociality is explored in [5].
Data or social media portability16 is an initiative aimed atproviding open standards for discovery, import, export andsynchronisation of user profiles, relationships, content andmedia. SIOC and FOAF, combined with domain specificontologies, allows to describe most of such information andcan form a solution for social media portability in an open
and machine-redeable way (Fig. 4).

5.

CONCLUSION

SIOC data created by online community sites are highlyinterlinked and ready for weaving into a larger Web of Data.
In this paper we described some approaches for facilitating
the linking to SIOC data and for using SIOC to link backto other RDF data.
14

http://sioc-project.org/node/262
http://rdfs.org/sioc/types
16
http://www.dataportability.org/
15

While some may consider social media content just an ”endpoint” in a journey for exploration of linked data, interesting
possiblities arise when these content items are both linked
to and contain links to other RDF data. In this case social
media content and associated SIOC data act as a linking
point by connecting together different parts of the linked
data universe.
The authors are looking forward to feedback and suggestions
from other implementers of Linked Data on the Web forenabling interoperability and reuse of the SIOC data andtools described here.

6.

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