Facebook has included a secret feature that not many people are aware of built directly into Messenger. The game can be played through the mobile app or the web browser and is within your chat window.

Using just a simple set of commands, You can play chess with your friend right on Facebook Messenger. To begin a game, simply open a chat with a friend and type “@fbchess play” which will initiate the game. Its important to remember that all commands related to the game will begin with “@fbchess” followed by the action.

Source: Techie Avenue
Facebook Messenger’s Secret Chess game

To move your pieces around, you can use standard algebraic notation based on the grid that is displayed on your board. To learn the commands simply type “@fbchess help” and you’ll get the full list of commands.

Here are the complete list of commands that you can use:

“Start game with random colors: @fbchess play Pick the colors: @fbchess play white/black Pick the opponent: @fbchess play white John Make a move: use Standard Algebraic Notation @fbchess e4 or @fbchess Pe4 moves pawn to e4 Nbd2 to move knight from b-file to d2 B2xc5 to take on c5 with 2nd rank bishop e8=Q to promote pawn to queen 0-0-0 or O-O to castle Claim draw (e.g. 3-fold repetition): @fbchess draw claim Offer a draw in the current position: @fbchess draw offer Offer an undo of the last move: @fbchess undo Resign: @fbchess resign Show current position: @fbchess show Show stats between current players: @fbchess stats Continue a game from another conversation: @fbchess continue From 1:1 conversation, @fbchess continue with [friend] From group chat, @fbchess continue from [thread name]”

According to a thread on Reddit, the feature has been there for about a month or possibly even more but went undetected. So go ahead, have a game of chess. Good luck!