I keep only 22 friends, and treat Facebook like an almost-private journal. Deep personal conversations work if you use Facebook like that. It's up to the user.
(Yes, I know, plain text emails to hotmail/gmail recipients would probably be just as bad as plain text private messaging on facebook. Protecting privacy is so bothersome that my efforts are pretty token. But I do try... because I don't comprehend the ramifications of releasing so much information.)