I'm a fan of Facebook because it allows me to at least keep apprised of friends that I don't often see or hang out with. I'm not a "reach out and find out how you're doing" kinda guy so the periodic Facebook events help me stay informed. The unfortunate side effect of that is Facebook is now starting to filter out posts and feeds for things they don't deem "important" to me, which is great to reduce clutter but when we're talking about my friends, that's information I care about and want to see and don't want a machine determining what is and is important or who I am and am not close to.
I use Twitter almost exclusively to follow WoW game developers and the New Jersey Devils beat writer. I know how Twitter works but I really don't get the social system. If I want to follow someone, or read a conversation between people, I want to be able to follow it (forum style), I don't like the fact that it's difficult to follow conversations on Twitter, replies are sometimes lost if it's popular enough or that if I have more than like, 10 accounts, the posts come so fast that I get information overload (most of it non-sensical garbage). It's cool that it's sort of a social link to celebrities and "high profile" figures, but I just don't see why it's popular with people who don't have anything meaningful to say. *shrug*