Someone I know quit FB at the age of 22, when his grandma tagged a girl that he liked and commented "you'd make such a great couple" or some such. Age separation is a big deal.
I think the biggest place where facebook fails, is making it easier to segment people and create groups.
Where you can easily setup a group, of say friends, and you can more openly post. Without telling grandma about how you dropped LCD at the concert two days ago and had a wild time.
You _can_ do this now, but it's clunky and painful.
^badjoke