Shares of Facebook (Nasdaq: FB) resumed their fall Thursday after director and early investor Marc Andreessen cashed out a big position.

In late Thursday trading, Facebook fell 37 cents to $21.99 after Wednesday's unexpected surge of 13 percent despite the availabilty of 800 million new shares of the Menlo Park, Calif., company eligible for sale by employees who had been barred from selling since the May initial public offering.

On Tuesday, before the big run-up, the Andreessen Horowitz venture capital firm sold 4.6 million shares of Facebook, while it retained an additional 3.9 millon shares. A representative explained the sale was attributable largely to Facebook's acquisition of photo app specialist Instagram last quarter for about $300 million wotth of shares; Andreessen Horowitz had been an Instagram investor.

Principal Marc Andreessen, 41, is a director of Facebook as well as of Hewlett-Packard Co. (NYSE: HPQ), the No. 1 computer company. He's friendly with co-founder Mark Zuckerberg, 28, and was among his early venture investors.

Andreessen was the inventor of the Mosaic Web browser while still a student at the University of Illinois. That was the basis of Netscape Communications Corp., one of the pioneers of the first Internet boom.

His Menlo Park, Calif., investment firm, which has taken investments from New York Mayor Michael Bloomberg, was also restricted by so-called “lockup” provisions from selling until this week.

Other firms, such as Accel Partners, sold part of their holdings in the IPO and several early investors, including Peter Thiel and Dustin Moskovitz sold their entire investments earlier this year.

Zuckerberg, who cashed out about $3 billion worth of Facebook shares in the IPO, has promised not to sell more until sometime next year.

Facebook's market capitalization is now about $48 billion, or 42 percent below its value in the IPO.ff