Mugshot is an open-source social networking application and developer platform created by Red Hat. The project features desktop applications for Windows and Linux (OS X is partially supported) and server software if you would like to build your own system nodes.
Mugshot features link sharing and collaboration (a link swarm), and music and TV tracking, sharing, and recommendations among other things. My afternoon’s a bit too busy to dig into the platform, but if you’re interested in open social networking features built on top of XMPP, Firefox, and GNOME check out the Mugshot developer wiki for more information.