I'm on Facebook, but I use it infrequently. It's too impersonal for me; for socializing, I prefer a nice little forum.
It's useful as a central clearinghouse of friends and family, though. I have people on there from two old jobs and my current job, various acquaintances from online forums and a bunch of relatives, some of whom I only see a couple of times a year in real life. So it's good to have if I have an announcement that I want to get to everybody, and it's a good way for people to get a message to me.