Marc Andreessen, founder of Netscape, Opsware and Ning and the former CTO of AOL, is adding a new notch in his belt: he has joined the board of directors of Facebook, two sources close to the company confirmed to us (speculation about Andreessen possibly joining the Facebook board started last month on the Boomtown blog). The company should be announcing it shortly, perhaps this week.

Andreessen will join Mark Zuckerberg, Peter Thiel and Jim Breyer on Facebook’s board.

At first glance, the move isn’t a fit for competitive reasons – Andreesseen is involved full time at Ning, a platform for the creation of social networks and arguably a direct competitor to Facebook. But Andreessen is known to be a mentor to Zuckerberg, who calls on him often for personal advice. It isn’t surprising that Zuckerberg would ask Andreessen to have a formal involvement with the company.

Andreessen will reportedly remain with Ning full time and retain his Chairman title.

The board position Andreessen has taken is one of the two vacant common stock seats Zuckerberg controls in addition to his own seat. One of those seats has never been filled. The other was previously held by former Facebook President Sean Parker, who left the company in late 2005.

Andreessen joins Facebook at a crucial time in its growth. Competitors MySpace and (increasingly) Google are gunning for control of the social graph, which may be the engine that drives the next big growth wave in advertising. Several early Facebook executives have left this year as the company has evolved, and new executives have taken their places. Zuckerberg needs a trusted guy in his corner to help him avoid missteps like the launch of Beacon last year, which has led to serious privacy concerns. Andreessen has fought similar battles in the past and won, and his counsel is clearly a competitive advantage.

Photo credit: Mathieu Thouvenin

Update: This has now been officially announced.