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.