At the risk of stating the obvious, Facebook and Twitter are massive, and allowing users to share your content via these sites is an absolute must if you want to maximise the viral possibilities of any game.
We can embed your game as a Facebook "app", integrate Facebook functionality into a game as an integral part of it (allowing players to call upon their Facebook address books and photos as part of the game), or simply allow users to share their progress in the game via Facebook or Twitter.