Depends on how you use it. To me, facebook is one of the best internet ideas of the last years.
I use it to keep a list of contacts. People I have there are not necessarily my friends, they're people I've met personally at least once and kept a conversation with at least once. THEN, if I ever in the future want to get in contact with them again, I have facebook. Think about it, how many people have you met in the past, friends of friends, in parties or wherever, that you've thought about later in your life but they're forever lost? The opposite also happens: I've started to notice that for instante when I go to a party, if I add everyone I chatted with, a few months later I wouldn't remember I had talked to some of those people if I hadn't added them. But if I add them I can go thru the list later and say "wow, I totally didn't remember this person existed". There's another subtelty which is sometimes I don't feel confortable asking a girl for a number in a certain social situation (maybe I'm with another girl there for example), but asking for her facebook contact is a lot more inofensive, especially if you ask for everyone else's, which I would do anyway.
STILL, I only have little more than 100 people (plus one internet celebrity) so yeah, I'm not as sociable as it could have sounded
Bottom line: if you think about facebook as a list of contacts, it's the greatest thing since e-mail. To me it's a really useful tool, not an e-penis competition like someone mentioned.
EDIT: Oh, and to answer the question directly, of those 100, I regularly socialize with about 30 (?) and exchange messages once in a while with about 5 other.