I mostly stay off of social networks, with the exception of HN, but other people put pictures of me on them anyway. I say don't put that on FB and people just laugh because they think I'm kidding, but what can you do?

Don't Facebook already construct "ghost" profiles of people who aren't on FB but are mentioned/tagged in others' content? So when you eventually sign up they already know stuff about you, in which case even abstaining is not enough!

Exactly, it's frighting how everyone is assuming they have to change their public persona in order to protect their online version!