I've been assimilated into one of the "social networks". I held out for as long as I could, knowing it would be a complete time suck and not sure if it was something I'd be into. I've rediscovered so many old classmates and friends. It's crazy. Some of them are from elementary/middle school - people I lost touch with after we graduated from 8th grade (parochial school). I've also found a bunch of my sorority sisters who I never expected to be in contact with. I have a tendency to lose touch with people once I graduate/move/whatever. I'm terrible at emailing and phoning.
I think my biggest reason for holding out, though, was that I love the blogging world. I regularly read a group of blogs that just blow me away. They are (mostly) written by women who are intelligent, funny, articulate, and are doing some fascinating things with their lives. I was afraid the networking would disappoint. It's definitely not food for thought, but it sure is fun.