Steve,
Like the others have said, Facebook have privacy settings that allow you to dictate what people can and can't see which goes a small way to ensuring your privacy.
Golden rule I suppose is to not post info you'd prefer to keep private, most of Facebook is cached by Google and other search engines. (Dependant once again on privacy settings)
Recently in a corporate security evaluation I was undergoing they used a statistic along the lines of "87% of recruiters search for you using Social networking sites".
I have nothing to hide, so on my facebook there are pics of me in "altered states" (before I had kids) and all kinds of things that perhaps potential employers would find objectionable.. but my privacy settings prevent this from being public info.. you have to be my "friend" to see me dancing like an idiot in a nightclub.
Bebo / Myspace.. don't bother.
Linkedin - I'd say a very professional social networking site, far removed from Facebook in that it's not designed for you to post photos of your kids, or your 30th birthday etc.
Also what I have found is that your facebook "name" matters, I altered my name on facebook as I used to go by my mothers surname when I was a kid at school, later in life I used my Fathers surname (it's whats on my passport) so I combined the two on facebook to help people who knew me by either name.
Since doing so, It's totally confused Google, so searching for me firstname lastname just brings up various posts on newsgroups and mailing lists and not Fb.
Anyway, it's worth it for keeping in touch with long lost friends, I use it to keep up with friends in New Zealand. (I'm from NZ, but live in Scotland) - heck even my mother has a FB page. (Allthough she has not updated it.. too challenging apparently.)
My two cents!
Cheers,
Nigel.