Facebook Inc director Peter Thiel sold roughly $400 million worth of shares in the Internet social networking company last week, cashing out most of his stake, according to a regulatory filing.

Thiel sold his shares on Thursday and Friday at average prices ranging between US$19.27 (S$24.2) and US$20.69 per share after the end of the first lockup, which barred early investors and insiders from selling shares following the initial public offering.

The sales, in which Thiel sold roughly 20 million Facebook shares, were conducted as a result of a trading plan that Thiel entered into on May 18, according to the filing.

Thiel, who co-founded PayPal and was among Facebook's earliest backers, still owns roughly 5.6 million shares of Facebook.

A spokesman for Thiel declined to comment on the sales.

Accel Partners, a Silicon Valley venture capital firm that was also an early backer of Facebook, distributed roughly 57.8 million Facebook shares to the limited partners and general partners of its various funds on Thursday, according to another filing.

The move allows those partners to sell or hold on to their distributed Facebook shares as they see fit.

Facebook, founded by Mark Zuckerberg in his Harvard University dorm room, became the only US company to debut with a market value of more than US$100 billion.

But investors have grown disillusioned with Facebook's inability to articulate a plan to reverse slowing revenue growth.

Shares of Facebook finished Monday's regular trading session at US$20.01, down nearly 50 per cent from their US$38 offering price in May.

More than 1.4 billion additional shares held by early investors and Facebook employees are set to become available for trading by year's end, as additional post-IPO lockup restrictions are lifted.