It is a burgeoning area. In December, Digital Sky Technologies bought into Zynga for $180 million. EA snapped up PlayFish for $400 million and Playdom, whose "Social City" game racked up 10 million players in about a month of existence, scored a $43 million series B.
Most social games as well as some casual games make use a business model of selling in-game "currency" for the purchase of anything from fertilizer to a straight-razor and combining that with player-privileges sales and advertising.
via readwriteweb.com