This week, Moskovitz has sold a total of 450,000 Class A shares at prices ranging from $19.19 to $19.22 a share, reducing the Facebook stake of his Dustin A. Moskovitz Trust to 6.15 million A shares, a filing with the Securities and Exchange Commission showed.

His Dustin A. Moskovitz Trust also owns 106.8 million B shares, which can be converted to Class A shares on a one-to-one basis at any time. Class B shares carry more voting power.

Moskovitz has been selling shares at the rate of 150,000 a day since August 19, the first day since Facebook's initial public offering that certain insiders were allowed to sell shares.

Facebook's IPO has been one of the most troubled in recent memory, with the shares debuting in May at $38 but quickly falling. On Wednesday, shares in the social network were trading around $19, beset by concerns over issues ranging from its mobile strategy to its slowing growth.

Moskovitz, a roommate of Facebook chief Mark Zuckerberg's at Harvard University, left the company in late 2009 to work on his own start-up, workplace software-service Asana.

Several Facebook insiders, including early backer Peter Thiel, have been selling shares at a rapid clip. Thiel, who sits on Facebook's board, sold 20 million shares earlier this month.

(Reporting by Sarah McBride; Editing by Chris Gallagher)

• A Permanent Economic Emergency