In a deal that has been in the work for weeks, Disney announced today that it would buy Playdom for $563.2 million, subject to certain conditions, as well as a performance-linked earn-out of up to $200 million.

Playdom, the second-largest developer on Facebook, successfully transitioned over the last year from its home base on MySpace, and now pulls in a total of 42 million “estimated active players” per month, according to the press release — 39 million of those are on Facebook, according to our AppData service.

Its investors have hinted at an initial public offering since last year, but we’ve also heard that the company has been busy talking to interested media companies, including News Corp, earlier this year; TechCrunch and VentureBeat reported the pending sale to Disney, last week. This deal has been in the works for at least four or five weeks, and may have closed weeks ago, according to an industry source we spoke with.

Chief executive John Pleasants, a veteran game executive, “wowed the Disney board with a presentation on social gaming at a recent company retreat,” according to The New York Times, and now he “is expected to work to develop new titles based on Disney characters and franchises.”

Pleasant’s presentation no doubt showed off the company’s big in-house hit this year, Social City, as well as Bola, a soccer game developed by Three Melons, a company it bought a few months ago. The two titles are the best results so far from the company’s build-and-buy strategy, which has included seven acquisitions so far in 2010 and numerous new games.

One of Playdom’s biggest rivals, Playfish, was bought by Electronic Arts last year in a deal worth up to $400 million, a watershed acquisition in social gaming that until today had been the largest purchase in the emerging industry. Playdom’s larger rival, Zynga, is valued up to more than $6 billion, we’ve heard from industry sources, it has raised hundreds of millions of dollars. It is on track to make $500 million this year, according to our Inside Virtual Goods report, although it’s possible that the company will make more, as some reports are indicating.

Disney had already been moving closer to Playdom lately, with subsidiary ESPN signing a co-development agreement with the game-maker in May, and with investment arm Steamboat Ventures joining in a $33 million second portion of its first round this past June. Disney Interactive Media Group President Steve Wadsworth tells PaidContent that Steamboat had participated in the latest funding “without any thought that we’d buy out this company,” but explains that since the investment, the two had gotten “a lot closer.”

The company has raised a total of $76 million in venture funding, reportedly at a single, $260 million pre-money valuation. The acquisition is expected to close by the end of Disney’s 2010 fiscal year, the company says.