From the amazingly intricate easter egg department: Facebook has a hidden, fully functional, graphical game of chess embedded into its Messenger, and anyone can play.
To start a game, open a Messenger conversation with someone and type "@fbchess play". A chess board will show up, and your game has just started!
Unfortunately, the board is not drag-and-drop; instead, you need to send text commands to move your pieces across the board, with the board refreshing as a new static image after each move. The commands use Standard Algebraic Notation; in layman's terms, this means that P is pawn, N is knight, B is bishop, R is rook, Q is queen and K is king. To move a pawn from the starting position, for example, you'd type "@fbchess Pb4".
The game supports draws (type "@fbchess draw offer" to offer one), undo (type "@fbchess undo"), and you can resign with "@fbchess resign".
There's quite a bit more to it; to get the full list of commands, type "@fbchess help" into Messenger.
The feature, which hasn't been publicly advertised anywhere, has been around for at least a month, when it was spotted on Reddit; the news passed relatively unnoticed until it was picked up by Lifehacker Thursday.