Walt Disney Co will acquire start-up Playdom Inc for $563.2 million, as the media giant aims to take a leading role in the fast-growing market for games played on sites such as Facebook and MySpace.

The deal is the richest ever in the young but fast-growing market for social games, which are free to play and appeal to a wider range of users than traditional video games.

The deal, which is expected to close by the end of Disney's 2010 fiscal year, includes an additional performance-linked earn-out of up to $200 million, Disney said on Tuesday.

Playdom is the fourth-largest developer on Facebook when measured by active monthly users, according to analytics tracker AppData.

Playdom draws an estimated 42 million active players each month for games such as Mobsters and Social City.

Disney's expansion into the still-evolving market for social games comes as it seeks to sell other businesses like its Miramax film unit. The company is refocusing on big franchise films and brands.

Janco Partners analyst Mike Hickey was surprised at the high price Disney agreed to pay for Playdom, but said it shows how serious a business social gaming has become.

When you look for growth in the near and medium term, it's digital online that will likely be the real contributor, Hickey said.

It definitely gives a bit more credibility to the market opportunity, he said.

The valuation on social gaming companies appears to be soaring. Last year, Electronic Arts agreed to buy Playfish, then the No. 2 game maker on Facebook, for $275 million in cash plus other consideration. Playfish at the time reported more than 60 million active monthly players.

Excluding potential earnouts, Disney is spending more than $13 per user to acquire Playdom, well above the almost $4.70 per user that EA paid for Playfish.

But the giant in the social gaming space remains Zynga, one of the hottest tech start-ups to come along in years. The company boasts more than 200 million monthly users on Facebook.

Zynga, whose venture capital investments give it a valuation of more than $4 billion, has been mum on its future plans. But many analysts expect the company to pursue an initial public offering as early as next year.

GROWTH BUY

Disney emphasized the growth potential it sees in online social games.

Social games are free to play and generate revenue by charging users to buy virtual goods, such as clothes, furnishings and weapons.

This acquisition furthers our strategy of allocating capital to high-growth businesses that can benefit from our many characters, stories and brands, Disney Chief Executive Bob Iger said in a statement.

Playdom, which has 15 game development studios, will remain headquartered in Mountain View, California. Playdom's investors include Lightspeed Venture Partners and Norwest Venture Partners.

The company has raised a total of $76 million in funding.

In June of 2009 John Pleasants, then EA's chief operating officer, raised eyebrows in the gaming industry when he left the company to become Playdom's chief executive.

We are at the start of a once-in-a-generation opportunity to transform the way people of all ages play games, Pleasants said in a news release.

Pleasants will become an executive vice president of the Disney Interactive Media Group and general manager of Playdom.

The Playdom deal follows Disney's purchase, earlier this month, of iPhone app developer Tapulous for an undisclosed amount.

Shares of Disney closed down 0.4 percent at $34.28 on the New York Stock Exchange.

(Reporting by Gabriel Madway; editing by Carol Bishopric, Gary Hill and Bernard Orr)

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