I used to avoid MSN Messenger because I would get too easily overwhelmed. You'd be having a conversation with one person, and suddenly wham! Everyone you know is online, and they're saying "Hi" to you, and I can only handle talking to one person at a time. (Now I avoid Messenger because I really don't like small talk, and I am incapable of ending a conversation there, for as long as I'm still online.)
In a different way, I'm starting to become overwhelmed by Facebook. I'm becoming overwhelmed by exactly what has excited me about the site: I can get in touch with people. Suddenly I'm tracking down (and being tracked down by) people from high school! People from undergrad! All sorts of camp people! Relatives! People from the Europe trip! My entire department here! So many different worlds, all congregated in one place. At this point, there are seventy-five of them.
It's like the online version of standing in the middle of a huge group of people, all of whom know you from completely different contexts. How do I act? Will something I say on my profile, which would appeal to one of my groups, make other people think I'm a crazy person? Why can't I just be funny like Ky?
It's not like blogging, where I can just think out loud and then people can respond to what I've said (and those people are either complete strangers or people who are close enough to me that I've given them my URL). Facebook is more like a public social situation where I might say something awkward or stupid. And I'm socially awkward like you won't believe! Especially among people who are in that scary middle ground between close friends and strangers.
(But that's not to say that I'm going to be avoiding Facebook anytime soon. It's just too fascinating to observe the complicated social web of those around me.)