The Number Game
Hot on the tail of the "Change your profile picture to a cartoon character to fight child abuse" Facebook game is the "Numbers Game".
This one isn't unique to Facebook alone; Twitter is also in the midst of this one as well. The concept is simple:
Your friends send you a private inbox message (or DM for Twitter) with any random number of their choosing, and then you post a status message (or tweet) starting with that number followed by what your really think of that person, or something personal you'd really like to say to them that you don't think they know yet. Since it's only a number no one else on your friends list really knows who it's about other than you and the friend who sent you their number.
Alot of people seem to be getting irritated with this one, are you too?