I'm on facebook for family and to keep in touch with people on a "geek" forum who seem to be moving from the forum to FB. I have friends that post interesting links about science and politics (and funny stuff).
I tend to hide "game announcements" like "I just planted a tree in Farmville" so I don't get bothered with those kinds of posts.
You have to be careful about what you do there, because some applications harvest your information.