Face Book Fuck Up: when you hit "comment" or "send" without proof reading. Then you have to go back and correct it or delete and start all over.
Your profile pic is good, but it looks like, is good but it looks like you were drink. (comment) This is a FBFU; it's already out there and you look like and ignorant idiot to all your Facebook friends.
2. FBFU, is also used for Facebooks nasty habit of occaisionally deleting friends, or not allowing friend requests to go through, or any of all the other little negative anomalies facebook has become famous for.