Although he’s been rumored to have a seat on Facebook’s board for a long time, Facebook announced today that Silicon Valley luminary and current Ning Chairman Marc Andreessen has officially taken the fourth seat on Facebook’s board.

A long-time advisor to the company and CEO Mark Zuckerberg, Andreessen is known for writing the early Mosaic web browser, co-founding Netscape, and staying on as AOL’s CTO after it acquired the company. Andreessen later founded web infrastructure company Loudcloud/Opsware, which was acquired by HP, and then co-founded Ning.

Andreessen joins current Facebook board members Zuckerberg, Jim Breyer of Accel, and Peter Thiel of Clarium Capital and Founders Fund. David Sze of Greylock and Paul Madera of Meritech Capital Partners also sit as board observers.

Commenting on landing Andreessen for the board seat, Zuckerberg said, “Marc is an industry leader, and we’re fortunate to have him join our board. He has experience that is relevant to Facebook in so many ways: scaling companies that are experiencing extraordinary growth, creating successful technology platforms, and building strong engineering organizations. I know Marc will be a great mentor to me and our leadership team.”