I have felt this same way when looking at facebook, redhead42. Most of my friends that I had in high school are still together and hanging out 5 years after graduation, just without me... It really hurts to see those pictures of them having fun, and wondering, analyzing what things I did to lose them. I am terrible at keeping up friendships, which I partially attribute to being bipolar, and my lack of drive or motivation.
But friends come and go, and that's sometimes for the best. What helps me is logging off Facebook when I start getting depressed, because I know it'll just make me feel worse.

(Also what makes me feel better is thinking about how whatever those people in the pictures are doing probably looks WAY more fun than it actually is).