If you're a Facebook user, you've probably heard of someone who's had their account disabled. I personally know four people it's happened to, and none of them posted anything typically considered offensive like naughty photos, nor did they harass anyone or use the site to spam.
GetSatisfaction.com wrote an interesting article covering 13 reasons an account could be disabled. The list includes not using your real name when creating an account (no nicknames, only what's on your birth certificate is permitted); you joined too many groups (they don't want anyone being too well networked); you posted too many messages (they think you're spamming, even if you're just a social butterfly); you have more than 5,000 friends (no one should be allowed to be that popular apparently!)
One I think it's totally legit: You poked too many people!! An avid user for years, I STILL don't understand what the poking is for. How obnoxious, it should be punished!