I almost caved, the second time that happened. What stopped me was being prompted for my email account credentials, so Facebook could mine my more than decade-long correspondence for social graph data. I know enough about abusive relationships not to overlook an opening boundary test like that. So that was the end of my Facebook experiment. In any case, by that point the damage was probably done.
Someone else here said something about having his Facebook departure be a conversation starter. Doesn't always work that way. When I tell people about that email prompt, they just look at me funny and go "you know you can skip that, right?" Which, of course, I do know, but see above re: abusive relationships. Maybe I can talk my way out of taking a punch this time, but every time?
That metaphor doesn't seem to cut much ice with anyone, though. No idea why. Maybe it's a little over the top. Maybe people are just so accustomed to think about Facebook as part of the environment, and take it totally for granted, that it doesn't occur to them to regard what I'm saying as anything but incomprehensibly weird. Maybe I'm an obstreperous pain in the ass. I'm sure at least one of those is true. But who knows? I mean, I don't ask; I just steer the conversation somewhere more mutually enjoyable, because I go to bars to drink and enjoy talking with people, not to pretend to be Richard Stallman.