I find it a good way of keeping up with what friends are up to (especially those I see infrequently due to location), and also for arranging catchups/nights out etc and for sharing photos.
However, I'm quite restrictive about who I add/accept as friends, and I have my privacy levels set very high so that only friends can see stuff (and also set up so that I can restrict content that work colleagues see - though it's only a limited few that I have on Facebook). Facebook is great if you're aware of how visible or otherwise you are, but people often don't seem capable of getting to grips with this!