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Game developers on Facebook soon must accept the social network site's Credits -- a move expected to pad the company's coffers, since Facebook charges a flat 30% whenever users buy anything with the currency.

The rule, which goes into effect on July 1, will be the exclusive way users can invest their real money into games running on Facebook. Developers may opt to continue their own in-game currencies, such as FarmBucks or CityCash, but users must buy these funds in Facebook Credits, not U.S. dollars, British pounds, or Euros.

Today, 150 developers use Facebook Credits in more than 350 applications, said Deb Liu, a platform marketing manager at Facebook, in a company blog. This represents more than 70% of the virtual-goods transaction volume on the site, she said.

Although Facebook is not yet forcing developers to use Facebook Credits as their only in-game currency, the company is offering incentives to those that solely depend on this legal tender, said Liu. These developers will, for example, get early access to product features and premium promotion on Facebook, such as featured placement on the site's Game Dashboard, premium targeting for ads, and access to new co-promotion opportunities, she said.

Over the next five months, Facebook will work with those developers yet to adopt Facebook Credits, collect feedback to improve the product, enhance the user experience, and help developers increase their revenue on Facebook, said Liu. Users benefit since they will be able to use Credits on any game running on Facebook, she said. "With Facebook Credits, people enter their payment information once and can buy, earn, and spend safely across lots of different games," said Liu.

Some developers cite Credits as the source of increased revenue. "Facebook Credits have helped PopCap set revenue records in Bejeweled Blitz every week since launching. The company's average revenue per user is considerably above projections and our virtual coin reorder rates exceed 80%," said Michael Carpenter, VP of social product operations at PopCap Games, in a statement.

But not all games developers have embraced Facebook Credits. In 2010, Zynga -- developer of many of the site's most popular games such as Farmville, CityVille, and Mafia Wars -- was reportedly considering ending its relationship with Facebook because of the 30% Credits fee. By May, however, the two businesses reconciled, and inked a five-year deal.

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