Memory Match is a great game that combines concentration and Facebook friends. The goal is to match the images of your friends. Each turn you flip over two images and if they match they are removed from the board.
You can play in single or two player mode. The goal of single player mode is to match your friends in the fewest possible turns. In two player mode each player takes turns matching.
(A Facebook account is not required as the game plays great with the default content.)