Social game developer Zynga is set to buy up fellow Facebook game developer Serious Business, and move all its employees to its San Francisco headquarters. "We’re very excited to gain an experienced team that is committed to building social games," says Zynga chief operating officer Vish Makhijani of venture-backed Serious Business, best known for Friends For Sale on Facebook -- a game which presently claims more than 5 million monthly active users. Financial terms of the deal weren't disclosed. Adds Makhijani: "We are impressed with Serious Business’ entrepreneurial attitude, speed, and innovation in developing social games and welcome them to our team." Zynga, developer of FarmVille, Mafia Wars and other popular social network games, currently boasts 60 million active users, and says that Serious Business' innovations will be useful in improving the games it serves. Zynga has seen explosive growth in recent years, as one of the top three social gaming companies along with EA-acquired Playfish and Sorority Life creator Playdom. Late in 2009, Zynga picked up $180 million in investment toward its further growth. Following his company's acquisition by EA, Playfish boss Kristian Segerstrale said that he sees the social gaming space rapidly maturing, and that consolidation among companies developing games for networks like Facebook looked likely to be a major part of that evolution.