I have accounts on a few social networking sites -- Skype (which I'd describe as a social VOIP service), LinkedIn, Facebook -- each of which involves networking among friends and acquaintances. I've also checked out Gather and MySpace. Except for Skype, which I use for actual phone calls, my use of other social systems has been mostly responding to various requests from others to be added. While many of my colleagues are enthusiastic about the possibilities for relationship building, especially for public media, so far for me personally it's been more chore than treat. Perhaps that's just the natural result of it being early in the game.
Update 31 July 2007:
Wired editor Chris Anderson writes about Why I Gave Up on Second Life. Link: The Long Tail. --Dennis