Silicon Valley entrepreneurial billionaire Marc Andreessen -- who co-authored the Mosaic Web browser and co-founded Netscape some 15 years ago -- has now joined Facebook's board of directors, though he holds stock in its competitors.

Andreessen serves on the board of Open Media Network, a company that produces a combined Kontiki (VeriSgn) client and media player. He is also an investor in Digg, Twitter, and several other Web 2.0 start-ups.

But amid all of Andreesen's other business interests, the one that comes closest to Facebook's space is Ning, a Web site that lets people name and create their own social networks. A few of the social networks now on the site, for example, include "The Spill.com Movie Community," which features movie reviews with animated characters; "Streetball.com -- Basketball for the Hip-Hop Lifestyle;" and "Oozing Goo -- The Lava Lamp Syndicate."

Although many see Ning as competing with Facebook on some levels, the exact reasons why Andreessen is joining Facebook's board -- while still keeping his post at Ning -- remain to be seen.

"Almost any way you slice it, these two companies look like natural competitors, with very different approaches to the social networking space," wrote Denis Hancock in his Wikinomics blog, for instance. "I've personally heard a lot [of] speculation (or perhaps I should call it postulation) about a future where each of us will have a single online identity point which links directly to our other points of presence on the Web - could this be a step in that direction?" In his own blog, Andreessen took note of the differences between Ning and social networking sites such as Facebook and MySpace -- and the potential for co-existence -- way back in July of 2007.

"I think both approaches -- large proprietary walled gardens, and millions of specialized social networks created by regular people -- will coexist just fine, but the more people learn what social networking makes possible, the more interested they will be in creating their own worlds, their own social networks, around every conceivable need and niche: their families, friends, cities, companies, start-ups, classes, hobbies, interests, political candidates, nonprofit activities, and so on," Andreessen wrote.

"That's why my co-founder Gina [Bianchini] and I started Ning -- to see what we could do to make it as easy as possible for regular people, potentially in the millions, to build their own social networks for anything. We wanted to enable people to create their own worlds, not just join someone else's."

Yet in an entry only about a month before that, Andreessen used his blog to applaud Facebook's recent development of the Facebook Platform. "Metaphorically, Facebook is providing the ease and user attraction of MySpace-style embedding, coupled with the kind of integration you see with Firefox extensions, plus the added rocket fuel of automated viral distribution to a huge number of potential users, and the prospect of keeping 100% of any revenue your application can generate," according to Andreessen.

In any case, Andreessen has nearly always displayed a Midas touch around Web-related business interests. He first rose to international attention in his early twenties, when he became the poster child for Internet "overnight success stories" with the IPO in 1995 of Netscape Communications, a pioneering ISP that used Netscape Navigator as its Web browser.

Riveted by Netscape's success, Microsoft licensed Mosaic code from Spyglass, Inc., an offshoot of Andreessen's alma mater, the University of Illinois. Microsft used the Mosaic code to create Internet Explorer as a rival to Navigator. Andreessen had co-written Mosaic with colleague Eric Bina when both were were employed at the university's National Center for Supercomputing Applications (NCSA).

When AOL bought Netscape in 1999 for $4.2 billion, Netscape became AOL's CTO. Shortly afterward, he left to found Loudcloud, a Web hosting company that sold is hosting business to EDS in 2003 and became known as Opsware. Then, as chairman of Opsware, Andreessen oversaw its acquisition by Hewlett-Packard in September of 2007 for $1.6 billion.