Peter Thiel

(Credit: Declan McCullagh/CNET)

While this doesn't look great for those hoping that Facebook's stock starts rebounding, it's hard to accuse Facebook investor Peter Thiel of not being a Facebook believer.

Thiel, who in 2004 put in $500,000 to become Facebook's first outside investor, today revealed in an SEC filing that he sold about 20 million shares on Thursday and Friday at prices between $19.69 to $20.70. That comes to about $400 million -- an impressive return even if he did sell at a price almost half Facebook's IPO price of $38 a share. Thiel already cashed out $640 million in the IPO in May.

Thiel, who is on Facebook's board, still owns about 5.6 million shares, according to the filing. Most of the selling was by Thiel's venture fund, The Founders Fund.

CNET has contacted Thiel for contact and will update this report when we learn more.

Other insiders are believed to have sold last week as well. This disclosure comes after the expiration Thursday of the first so-called "lockup" period, which had restricted insiders with pre-IPO shares to sell their holdings. Last week's "lockup" covered about 271 million shares owned by insiders and early investors.

Facebook's stock took a beating on Thursday and Friday, although shares rose more than 5 percent today to close at $20.01. There are three more "lockup" periods coming up this year, with the biggest in November, when 1.32 billion shares could hit the market.