Dustin Moskovitz, 28, sold 450,000 shares in the company last week, according to documents filed with the US Securities and Exchange Commission.
The sale represents around 6% of his holdings in the firm that he helped set up with Mark Zuckerberg in 2004, netting him around $8.8 million (£5.5m).
Moskovitz - who left Facebook in 2008 to co-found workplace collaboration firm Asana - sold the shares in three 150,000 chunks last week.
He had converted 7m of his 'Class B' shares to 'Class A' ones. 'Class B' shares have ten times the voting power, but they need to be transformed into the 'A' version to be sold publicly.
In March 2011, Forbes ranked Moskovitz as the world's youngest self-made billionaire based on his 7.6% share at the time in Facebook.
Despite Moskovitz still holding an estimated 126m shares in Facebook valued at $2.4 billion, his decision to sell now is viewed as significant, coming at a time the company is facing increasing scrutiny.
Earlier in the week, it emerged that early Facebook investor Peter Thiel had sold the vast majority of his stake in the firm after the end of the first lock-up period, in which Facebook's founders and initial backers were prevented from selling their shares.
PayPal billionaire Thiel, who also sold a batch of shares in Facebook when the company floated in May, has made almost $1bn dollars from his initial investment in the firm in 2004. He remains a Facebook director.
Following the botched IPO and struggles ever since to convince investors, there have been calls for Facebook co-founder Mark Zuckerberg to step down as chief executive of the firm.
"There is a growing sense that Mark Zuckerberg, talented though he may be, is in over his hoodie as CEO of a multibillion-dollar public company," Sam Hamadeh, head of research firm PrivCo, told the Los Angeles Times.
Meanwhile, Facebook yesterday (August 22) got the go-ahead from US regulator the Federal Trade Commission for its acquisition of photo-sharing app instagram to go ahead.