Facebook today announced that Web pioneer and Netscape founder Marc Andreessen has joined its board of directors. Currently co-founder and chairman at Ning, Andreessen is an investor in several Internet startups. A prominent Silicon Valley veteran, Andreessen co-founded Ning as a complementary platform to Facebook that allows people to create their own social network with any design, any feature set, and in any language. He also authored early Web browser Mosaic and was co-founder of Netscape Communications. Andreessen’s experience also includes his work as CTO of AOL, as well as chairman and co-founder of Web infrastructure company Loudcloud, later named Opsware and acquired by Hewlett-Packard. According to Zuckerberg, Andreessen joins current Facebook board members Jim Breyer of Accel Partners and Peter Thiel of Clarium Capital and Founders Fund. Facebook board observers are David Sze of Greylock Partners and Paul Madera of Meritech Capital Partners. 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. Andreessen will become the fourth member of a Facebook board that includes Zuckerberg and venture capitalists Jim Breyer and Peter Thiel.