Zynga has acquired fellow social game maker Serious Business for an undisclosed sum, reports Reuters.   Serious Business is best known for Facebook game Friends for Sale! said to be played by an estimated 6 million people monthly.  Reuters says the buyout comes as recently venture cash-infused Zynga is looking to expand its slate of social games.  Zynga claims more than 235 million people play its Facebook games every month.  The company pulls in an estimated $300 million a year in revenues from virtual goods sales in games such as “FarmVille” and “FishVille.”  Reuters says analysts have pinpointed Zynga as an IPO candidate that could raise more than $1 billion in a stock offering.  Read more at Reuters.