American Express and Zynga, the company responsible for addictive social media games such as FarmVille , CityVille, Words With Friends and Draw Something, have come up with a card that allows people to earn virtual rewards when they spend real cash.

"We're excited to partner with American Express to invent new ways for people to experience Zynga play in more parts of their day," said Mark Pincus, CEO and founder of Zynga in a statement. "Together we can add surprise and delight to everyday shopping."  

The new program is called Zynga Serve Rewards and functions through a prepaid card and American Express' Serve digital wallet. To use the digital wallet, users register with Serve, then they load up the wallet by tying it to a bank account, debit card or credit card. Users can also fill the wallet by spending cash on a MoneyPak that will transfer funds to the wallet.

"We're thrilled to expand our relationship with Zynga and provide an easy way for players to earn Zynga virtual game cash by learning about Serve and signing up to receive the many benefits of our digital wallet," said Dan Schulman, President of Enterprise Growth, American Express in a statement issued on May 22nd. "As the commerce landscape continues to change, and online and offline spending converges, Serve is focused on partnering with companies like Zynga to create unique value for our customers in the environments they love."

FarmVille is a popular interactive online game and Facebook application that simulates the fun of growing crops on a farm. Players cultivate their farms by plowing, planting and harvesting crops and trees. Their social networks help them with their farms, and as the crops thrive, so does a player's success in the game.

Through the Zynga Serve Rewards Program, players can plant an interactive Serve Money Tree on their farms. The tree allows players to level up in the game and earn Zynga Farm Cash, which they can redeem for virtual gifts that help them with gameplay.

Zynga and American Express are slowly rolling out the various types of benefits users can get with this new card. The initial phase of the program runs through July 31, 2012. To take advantage of the rewards, players will need to register for the program. Once registered, they'll receive the co-branded card in the mail. Registrants should also add money to their Serve digital wallet account to facilitate usage of the account. When the card arrives, users will activate it, and then they'll be able to use the card for purchases.

Cardholders can use the Zynga Serve Rewards cards to purchase items anywhere American Express is accepted in the U.S. In the initial phase of the program, users can only earn Farm Cash rewards on their first five purchases of $25 or more. In return, users will receive online, in-game rewards in real time.

During subsequent phases of the card program, users will be able to track their rewards from purchases through an in-game dashboard. Zynga also plans to add other kinds of loyalty incentives that allow cardholders to link their daily spending to Game Cash in ways other than using the first five purchases of $25 or more.

Zynga Serve Rewards launched this month. Currently, rewards can only be applied to FarmVille, but Zynga plans to add similar rewards for some of its other games. It intends to add CastleVille and CityVille later this year. Other planned elements of the program include new kinds of loyalty incentives and expanded ways to use everyday spending to earn Zynga Game Cash.