This is a newish site in beta that combines fitness/exercise with social networking and a "game" element (you log your workouts for points/levels/quests, that your friends then see). I've only been on it a few days but it's a lot of fun and very motivating. I've got a few invite codes if anyone wants to try it out. PM me your email address if so.

(Not affiliated--site doesn't appear to be monetized yet anyway, although I have a feeling it'll explode soon.)