Facebook's chief executive Mark Zuckerberg has seen the value of his personal shares in the site halve, after they fell from $19.1bn (£12.2bn) to $9.9bn (£6.3bn).
The social networking giant's shares fell 6% on Wall Street last night to set a record low below $20 (£12.70) after some investors who bought into the company before it went public were allowed to sell up to 271 million shares.
As a company Facebook has now seen some $49bn (£31.2bn) wiped from its value since its $104bn (£66.2bn) flotation in May, when it was priced similarly to giants Amazon and PepsiCo at $38.
It will not be known until at least next week which of them offloaded stock, but more than 156 million shares were traded yesterday - more than five times the stock's average volume over the past month.
Despite the sharp drop in Facebook's market value during the past three months, the early investors, who picked up shares as cheaply as $5 a share, can still reap huge windfalls by selling at the current price.
Since launching on the stock market, Facebook has come under intense pressure, not least from rivals such as Google and Yahoo.
In June, Ironfire Capital founder Eric Jackson predicted: "In five to eight years they are going to disappear in the way that Yahoo has disappeared."
But some analysts believe the value of shares will return once the market has settled in a few months time.