Facebook is adding a Discover New Games sidebar module that shows off what games users are playing. It is designed to be a way to grow games outside of developer promotion and revive some of the virality lost by the current set up of Facebook.
The module displays the names and faces of the viewer s friends who play a game the viewer has yet to install. The name of the game, its thumbnail icon, and a ‘Play Now’ link appear below the facepile. When users see this implicit social recommendation by friends, they assume their must be some entertainment value in the displayed game, and may choose to click and install it.
Source: Inside Facebook