I find it useful, it's entertainment for when I'm bored, can keep up with what my mates who don't live near me any more are up to, it feeds ice breakers, it's a one stop shop for arranging meetings and rugby meets and I've just started my little mess about micro-blog which keeps me creative!
I also met my lass thanks to facebook (perving at her on her mate's photo).
I use it mainly in the same way I use this site, a little bit of casual social interaction throughout the day, no harm in that!