I'm on there because our family is spread out all over the country, with some of my wife's cousins and aunts are living in other countries as well, so it's a great way to keep them updated on our young children. There are only 2 school friends who are my Facebook friends (the rest were dorks then, so why would I want to be friends with them now?), but I have used the groups function to stay in touch for high school reunions etc
I use Myspace and Facebook for promoting my bands, gigs, and networking with other musos, and have picked up several well paying gigs and cushy fill in gigs via the networks I've developed on both sites.
I'm a whore on Myspace, in that I'll befriend anybody, but on Facebook my 100 friends are either "real life" friends, family or long time imaginary internet forum friends. That's the line I draw between the two sites.
Facebook may be a necessary evil, but it's been profitable for me.