I'm not one of these addicted folks who spend entire days there, but I log in about once or twice a day, not for long though. I mainly use it for talk with some friends, but only the ones I really know. I have about 35, give or take, whilst many of my friends have 1325... stuff like that. I doubt they know 3/4 of them. Anyway, my main method of communication with friends is MSN, I only chat in fb with people whose MSN I don't have.
I don't plan to delete my account. It's certainly not the best thing ever (though 100x better than hi5, darn, that was such a failure, and a crap), but IMO worth keeping around. There are actualy some NASA, Roscosmos news I found there, and didn't find anywhere else.
EDIT: Plus, that last paragraph in Eli's post reminded me, next year I'm going to a High School where I know nobody, possibly only one person, so it will be usefull to keep in touch with old friends.