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The popular online game uses two units of in-game currency: Farm Cash and Farm Coins. Previously, you could buy them with a credit card or PayPal. Now the game offers Facebook Credits as an option.
Facebook Credits are the number-one option, actually. They're the default payment choice, featured at the top of the list pictured here.
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We recently learned that Facebook takes 30% of developers' Facebook Credits revenue — the same percentage that Apple takes from its iPhone and iPod touch App Store sales. Analysts have speculated that Facebook might surpass $1 billion in revenue this year, and the social network's virtual currency could be a vital part of growth beyond that.