© Reuters. Zynga founder Mark Pincus will return to his former position after departure of former CEO Don Mattrick

Investing.com -- Shares in social game site Zynga Inc (NASDAQ: ) plummeted more than10% in after-hours trading after Chairman of the Board of Directors Mark Pincus sent a note to employees announcing the departure of CEO Don Mattrick and his return to the position.

Pincus founded the company in July, 2007 and held the CEO position for six years until he was succeeded by Mattrick 21 months ago. The company is known most for its Farmville game, which garnered more than 10 million daily users within weeks of its launch in the summer of 2009.

"Don joined us at a very important time in our evolution. In less than two years under his guidance, our teams have worked hard to better serve our mobile players and deliver world class quality and value to our consumers," Pincus said in the note. "This hard work for our mobile players has resulted in bookings growing from 27% mobile when Don joined to 60% by the end of last year."

Farmville and the company's other top games are widely available on mobile phone platforms such as Apple (NASDAQ: ), Android and Windows Phone. The company has plans to shut down two popular games, CityVille and CastleVille, at the end of the month.

The anticipated removal of the games from app stores represents a shift in the company's reliance on some of its older games.

"Now that we are a mobile first company, it’s time to renew our focus on our vision to make play and social games a mass market activity." Pincus added.

Shares in Zynga plunged 0.31 or 10.69% in after-hours trading to 2.59.