Zynga's (ZNGA) social games — especially its hit Facebook (FB) game FarmVille — are a big deal in digital payments. Its games are the key to the success of Facebook Credits, and they may provide a similar boost to American Express' (AXP) Serve.

Facebook gets about a 15% of its revenue from payments, according to its regulatory filings, which say that "apps built by developers of social games, particularly Zynga, are currently responsible for substantially all of our revenue derived from payments."

Now Amex is getting in on the game.

Its deal with Zynga, announced May 22, ties the Serve digital wallet closely to FarmVille. Though the companies have worked together since 2010, their new agreement provides for deeper integration — activating a Serve account is actually a quest in the game.

"It is really embedded into the gameplay," says David Messenger, Amex's executive vice president of online and mobile, in an interview. "It's the first time Zynga's ever engaged with a third party in this way."

Players begin the quest by planting a Serve Money Tree in their virtual farms. The next steps: open a Serve account, fund it, activate it, and then use the attached prepaid card to make purchases and earn rewards.

Users can redeem the reward points for Farm Cash, which they can spend in the game.

Amex introduced Serve as a way to attract customers outside the market for its credit and charge cards, which it has traditionally marketed as elite products.

Though Amex has been aggressive in forging partnerships with social media sites, it still has a lot of catching up to do, says James Van Dyke, the president and founder of Javelin Strategy and Research, in an interview.

Rivals such as Visa (NYSE:V) have leapt ahead by purchasing companies that were built around social media and gaming.

"It's tough, they're up against a company like [Visa], with PlaySpan, where Visa actually acquires a company and brings it in-house," Van Dyke says. Visa bought the online payments company PlaySpan last year.

Read the full story at PaymentsSource.com