I'm guessing, based on past threads/posts, that a lot of people here aren't on facebook or use twitter.  
Personally, I have a number of friends on it and my wife is on it quite a bit.  I have a good friend and we basically share the same philosophy about it:  I stay in contact either by hanging out with, talking on the phone to, or emailing friends that I want to be in contact with.  Having someone from high school, college, or past employment by my friend on facebook is nice but if I really wanted to contact them, I'd have done it by now in a more personal way.  I think its ok for very casual interactions but not for me.