Short quote: Garbo
Because I have acquaintances* (not friends), barely talk to anyone I'm related to, and have almost no interest in strangers. That's why I'm not on Facebook. For me, the words "social" and "networking" are like shoving a cross made out of garlic in the face of a vampire:
True, you might not want people to be able to follow your life—it's no great loss to you if your long-lost college frenemy can't find you. But what about your old fling, your new fling, your next employer, or that friend-of-a-friend you just met at a party who says he can give you some great tips on your golf swing? Sure, you can trade e-mail addresses or phone numbers, but in many circles Facebook is now the expected way to make these connections. By being on Facebook, you're facilitating such ties; without it, you're missing them and making life difficult for those who went looking for you there.
I consider this a good thing: "...making life difficult for those who went looking for you..." I'm not an unfriendly person, I'm just trying to lead an uncluttered life.
*Not to disparage the few people I've met through this experiment of a blog; some of whom I'm sure would breach the acquaintance wall. Or at least I'd have a very enjoyable time hanging out with. But seriously, other than a Christmas card I've had no contact with my closest friend for about two years.