Avatars United: Dedicated to many MMOG’s and Virtual Worlds but has an interesting premise. One login for as many virtual world personalities that you may have.
Koinup: A social network dedicated to sharing art, machinima and virtual world stories. Koinup seems to have been dominated by Second Lifers but also includes others.
Rezzed.net: One of the smaller Second Life specific social networks. Rezzed.net includes a shoutbox, event listings, and a Status update.
SLProfiles: The oldest Second Life social network.
SLScout: Ran by the NingID system, SLScout seems the most AJAX of all the social networks here. It uses a single login (as many social networks on the NingID system)
SLUniverse: Although not a social network in definition, this community site is one of the longest running Second Life community sites.