The ESP game is an idea in Computer Science for addressing the problem of creating difficult metadata, which makes use of the computational power of humans to perform a task that computers cannot yet do (originally, image recognition) by packaging the task as a game. It was originally conceived by Luis von Ahn of Carnegie Mellon University. Here is an example of the game which supports image tagging: http://www.gwap.com/gwap/gamesPreview/espgame/. You will develop a game with similar functionality as a game on Facebook.