Peter Thiel, one of Facebook's [NASDAG: FB] earliest backers, has sold most of his nearly $400 million stake in the social network company.

Thiel traded in 20.1 million shares worth $395.8 million last week, the BBC reported.

News of the Paypal co-founder's exit comes a day after several of Groupon Inc.'s [NASDAQ: GRPN] largest early investors have sold their stock in the troubled online voucher site recently.

Internet finance mogul Marc Andreessen, who invested $40 million in the company months before it went public last November, cashed out, while investors Fidelity and Maverick Capital also sold a large portion of their shares.

Groupon's stock price has fallen by three quarters since its glitzy IPO, a loss of around $10 billion, and the firm's rapid fall from grace echoes the disappointment of other online stock failures such as Facebook and Zynga.

The exit of the Netscape founder's investment firm from Groupon is another blow to the beleaguered daily deal website, and combined with the Facebook sell-off, the sale compounds fears that another dot-com bubble is about to burst as it did in 2000.

Last week was the first time large investors could off-load Facebook shares since the company's stock market debut two months ago.

Last night, Facebook shares closed down at $20.01, almost half what they were in May.

Thiel, who owned around 10 percent of Facebook after a $500,000 investment in 2004, made a profit of around $1 billion on last week's sale and an earlier share trade in worth $640 million.

He still owns approximately 7 million shares.