Facebook Inc's (FB.O) first major investor Peter Thiel has sold three-quarters of his remaining stake in the social network as part of a previously established trading plan, a regulatory filing showed.

Thiel, who is a member of Facebook's board, had already sold more than US$1 billion worth of its stock before the filing made on Tuesday.

It said he had sold another 160,805 shares for about US$29 million, leaving his holdings at 59,913 Class A shares in the company.

The only major name in Silicon Valley to back U.S. President Donald Trump, some have called for Thiel’s removal from Facebook’s board. Chief Executive and co-founder Mark Zuckerberg has said he should stay, citing the importance of diversity of opinion at the company.

A Facebook representative called Thiel’s sales routine and said there was nothing new to say about the billionaire' s seat on the company's board.

Representatives for the venture capitalist could not immediately be reached for comment.

Thiel co-founded payment service PayPal and is known for funding the Hulk Hogan lawsuit that led to the shutdown of online news site Gawker. He became a Facebook investor in 2004 with an initial investment of US$500,000 at a US$5-million valuation.

When Zuckerberg took the company public in 2012, Thiel sold 16.8 million shares at the IPO for about US$640 million. Later the same year, he sold roughly 20 million of his remaining 26 million shares for US$400 million after the expiry of a lockup.

He hit the market once again in 2016 to sell a little less than one million shares for about US$100 million.

Facebook shares were down 0.7 per cent at US$180.59 on Wednesday.