Social Networks and the Semantic Web

FOAF

FOAF-a-matic by Leigh Dodds lets you create a FOAF profile for yourself by filling out a web form.
FOAFnaut by Jim Ley lets you browse the network of FOAF profiles via a nice visual SVG interface.
FOAFscape by Mark Giereth is browser for the FOAF network.
FOAFnet lead by Mark Canter is an industrial consortium of social networking services. They are adapting FOAF to be used as an interchange format between social networking sites (i.e. you can import your user profile from one system to another with the click of a button).

Flink: The Who is Who of the Semantic Web by Peter Mika. Browse the social network of Semantic Web reseachers via this FOAF-based website.
RDFweb image annotation experiment by Dan Brickley was the first experiment at image annotation using FOAF.
Tidepool by Immuexa (Tim Falconer) is a personal digital media organizer software that uses FOAF.
Plink is a social networking service based on FOAF.
PeopleAggregator by Marc Canter will be one day a social networking service based on FOAF.
SXIP Networks (pronounce: skip) start-up by Dick Hardt involved in the Digital Identity Management/Single-sign-on business, possibly adapting FOAF.
Trust and Reputation by Jennifer Golbeck. One of the works that derive trust measures from social networks (represented in FOAF).