Social gaming platform OpenFeint is saying goodbye to its 26-year-old co-founder Jason Citron, who has decided to leave the company to pursue new opportunities.

Citron will be replaced by Naoki Aoyagi, who will serve as the company's new CEO. Aoyagi was previously the head of Gree International, a division of Japan's leading mobile social gaming platform Gree. Back in April, Gree purchased OpenFeint for $104 million.

In just over 24 months, OpenFeint has grown to over 120 million users across 7,000 games, Aoyagi said. I thank Jason for his leadership growing the company and wish him well in his new adventures.

When Aoyagi joined Gree in 2006, he was the company's 10th employee. Now, Gree is expected to make between $1 and $1.5 billion in revenue this year alone, thanks to its 25 million or so Japanese users.

At Gree, we are socializing the next evolution of games and, as the best-in-class U.S.-based mobile social network, OpenFeint is the ideal partner for us to offer the best mobile social games to the largest global audience, said Gree CEO and founder Yoshikazu Tanaka in a statement made following his company's acquisition of OpenFeint.

OpenFeint, the Burlingame, Calif.-based start-up that launched in early 2009, is a unique platform that lets developers seamlessly add social networking aspects into their games, such as adding friends, chatting, forums, and live leaderboards. Available on iOS and Android devices, the system is relatively unintrusive, as developers only need minimal additional programing to add OpenFeint as an extra layer on top of the actual game.

One of Gree's prime competitors is Japan's DeNA, which actually used to be a part owner of OpenFeint. Like Gree, DeNA similarly acquired a California-based start-up in San Francisco's mobile games firm Ngmoco.

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