By most measures, Zynga -- creator of social games such as FarmVille and Mafia Wars -- is a success. The company is profitable and attracts millions of users each day. But the solid business has one potentially fatal flaw -- its dependence on Facebook's platform and user base. Can Zynga, which recently filed for an IPO, diversify beyond the social network? And does it need to, as the Internet world becomes increasingly interconnected?