Marc Andreessen has joined eBay’s Board of Directors, effective immediately. An entrepreneur, investor, and blogger, Andreessen’s current focus is Ning, a next-generation platform for social networking.

“Marc is a true visionary whose experience will be invaluable to eBay,” said John Donahoe, president and chief executive officer of eBay Inc. “We look forward to learning from Marc’s insights and expertise as we drive further innovation on our platform, invest in growth opportunities and develop technology that will further benefit our customers, build powerful communities and enhance e-commerce.”

Andreessen has served as an angel investor and advisor to startups including Digg, Del.icio.us, Twitter, Qik, Aliph/Jawbone, LinkedIn, Meebo, Netvibes, Outside.in, Revision 3, Fon, Funny or Die, Scribd, and Wikia.

Andreessen also founded Opsware, formerly known as LoudCloud, a provider of data-center automation software that was acquired by Hewlett-Packard in 2007. His most recent endeavor is as co-founder of Ning (he’s pictured here with Ning co-founder and CEO, Gina Bianchini), an Internet company developing a “next-generation platform for social networking” that allows anyone to build “your own social network for anything.”