NEW YORK - Zynga said on Thursday that it would buy Serious Business as the online game provider, often seen as a top IPO candidate, expands its stable of social games on Facebook.

Zynga, known for games including FarmVille and Mafia Wars, did not disclose terms of the deal for San Francisco-based Serious Business, whose game Friends for Sale! is played by more than 6 million people each month.

So called social games are seen as a growth area in the global interactive software industry, as million of users flock to the games, which are mostly free to start. Users typically pay small fees to add features to a game.

Late last year, Electronic Arts, a market-leading publisher of videogames like Madden NFL, agreed to pay $275 million in cash for Playfish, maker of the popular game Pet Society.

Zynga boasts more than 235 million monthly active users playing its games on Internet social networks like Facebook and MySpace, and profits by selling add-on tools. The game FishVille, for example, lets players buy virtual food and goods with game cash bought with real-world dollars.

Virtual goods account for 90 percent of Zynga's revenue, which has been estimated at $300 million on an annualized basis.

Analysts expect the technology sector to produce a number of initial public offerings in 2010, and have speculated that Zynga could raise more than $1 billion in an IPO.

(Reporting by Franklin Paul; Editing by Lisa Von Ahn)