My mom is on facebook.
Also, I have a relatively large family, many of whom are on Facebook. A year ago they were trying to use Facebook for family notices and communication. These days they've gone back to email. Everyone has email, but not everyone is on Facebook, not even among those who have already joined at one point.
As long as you can only do facebooking via Facebook, there will be people who aren't connected to it.
I mean, if facebook was providing me something meaningful I would have found a way to deal with it. But as it was I was mostly leaving my wall questions and my facebook mail unanswered; It really was better, I think, to leave rather than have people expect me to answer questions and then have those questions drowned out by mafiafarmcow updates and pictures of cute cats. (I know, you can filter some of the crap... but you know what? I already know how to use procmail. I already have a 200+ message a day email inbox, I don't need another.)
So the thing with my mom(s) was sort of the last straw. Everyone has my email; I went from mostly ignoring my facebook to deactivating it.
Now, my professional life, well, I'm on linkedin, and I like it quite a lot. But facebook has lost it's compartmentalization, and I think that's bad. It used to be just for college kids, which is a great niche, everyone else wants to be in the same place as the cool college kids. But if mom starts showing up at the local watering hole, I think the kids are going to find a new place to hang out.
The thing about linkedin is that yeah, I want to compartmentalize my professional life to some extent... but it's one way compartmentalization. I mean, while I don't really want all of my personal life to bleed into my professional life, I mean, my business partners and customers don't need to read about my relationship problems. Not that I'm ashamed of my personal life or anything; it's just unprofessional, and I think impolite to shove that in their faces. But the other way around is fine; I'm okay with casual friends reading up on what I do at work.
Really, the compartmentalization of my private life is very complex, and I'm not a very private person.
Just my opinion; I'm clearly not an expert at this sort of thing.