Ning CEO Steps DownNing CEO Steps Down

Ning chief executive Gina Bianchini has resigned from the social network builder she co-founded with serial entrepreneur Marc Andreessen more than five years ago.

Bianchini did not give a reason for her departure, announced by the company Monday. Her duties were handed over to Jason Rosenthal, who has been chief operating officer and head of business operations for the last year and a half.

Andreessen said on the Ning blog that Bianchini would become "executive in residence" at his venture capital firm Andreessen Horowitz. Andreessen's partner in the firm is Ben Horowitz, who co-founded the enterprise software company Opsware with Andreessen. The company was sold to Hewlett-Packard for $1.6 billion in 2007.

Ning provides users with the online tools to build their own niche social networks around any topic of interest, from the arts and business to politics and sports. The company claims to have 45 million registered users using more than 2.3 million user-created networks. According to Andreessen, Ning is growing at a rate of a million new registered users every 12 days.

Bianchini has been the face of Ning since taking over the company. In 2008, she appeared on the cover of Fortune on the magazine's list of the "most powerful women."

"Now that we have built Ning to its current position in the market, I am ready to hand the reins over to Jason and the team to continue to execute on our vision and strategy," Bianchini said in a statement. "As a founder and large shareholder, I'll continue to do everything I can to support and cheer on Ning in my new role!"

Bianchini leaves as Ning faces an increasing number of competitors, such as ProsperoTechnologies, The Village, WackWall, and a couple of dozen other companies. Rosenthal, however, has been with Andreessen for quite awhile. They worked together at Netscape, which Andreessen co-founded in the 1990s and sold to America Online in 1998. Rosenthal later worked for Andreessen at Loudcloud, which was later renamed Opsware.