I do, I signed up after being pushed into it, and now all it does is make me feel guilty every time I get a message, as it might be an invite, as some of my friends invite me to RL events thru facebook.
Then I log in, and find there's more people wanting to be my friend on facebook, I have about 200 unanswered messages to do with sheep and drinks and farms and the mafia, and I just feel like I'm being mean by not joining in, but it'd take me like a solid month to catch up, and I also just don't want my ex girlfriends, my family, my ex school friends, my current friends and my WOW guild mates all being linked together.
I just can't face the idea of my old sexual partners and my mother and 13 year old nieces and nephews sharing the same place. That's a Venn diagram where I want no part of either circle even being on the same piece of paper. Last thing I need is 'you've got a lot of girls on facebook, how come you're single?' from my mother, or 'who's that girl she looks fat have you done kissing with her and touched her boobies' from nephews and the like.
EDIT: Is it possible yet to completely delete a profile from Facebook? I heard you could leave, but your profile would still stay forever as they own you the moment you sign up. Personally I'd like to quit and just send everyone an email address to contact me.