The fortune of Mark Zuckerberg, Facebook Inc.’s co-founder and CEO, dropped to a new low Thursday – and even further on Friday.

The 28-year-old’s net worth fell to $10.2 billion, its lowest point since the company’s May 17th initial public offering, according to the Bloomberg Billionaires Index. Facebook (Nasdaq: FB) shares sank 6.3 percent Thursday after the company freed up 271.1 million shares, a move that boosted the amount of stock available to trade by 60 percent. The shares closed at $19.87 in New York after dipping to a record low of $19.69 earlier in the day.

On Friday, shares fell 80 cents, or 4 percent, to close at $19.07 in heavy trading. More than 115 million shares traded hands, nearly triple the dail average.

“The market is not convinced of Facebook’s future,” David Kirkpatrick, author of “The Facebook Effect,” a history of the company, said in a telephone interview, adding that Zuckerberg probably isn’t worried about the sagging value of his own shares. “He thinks of it only in terms of how it affects the company’s strategic and tactical opportunities.”

While Zuckerberg’s fortune swooned, the 40 richest people in the world added $1.9 billion to their collective net worth, as the Standard & Poor’s 500 Index closed at its highest point since April.

Facebook is down 48 percent from its $38 offering price. The world’s largest social-networking company raised $16 billion in its IPO, the largest ever for a technology firm. With a market capitalization of $48 billion, Facebook has lost more than $40 billion in value since the offering, making it the worst performer among all large IPOs on record, according to data compiled by Bloomberg.

Founders Falter

The fortunes of Facebook’s other founders also took a hit. Co-founder Christopher Hughes, 28, owns about 22 million shares of Facebook, according to a person familiar with his holdings who asked not to be named because the matter is private. His stake is worth $437 million, down almost $400 million since the IPO.

Dustin Moskovitz, 28, who started the company with Zuckerberg, Hughes and Eduardo Saverin from a dorm room at Harvard University, owns 133.7 million shares of the company’s Class B stock worth $2.7 billion, a decline of $2.4 billion.

Saverin, 30, has a $1.1 billion stake, down $960 million since the offering. According to a regulatory filing dated May 17, he owns 53.1 million shares of the company.

Facebook Inc. director Peter Thiel, who sold more than 16 million shares in the company’s initial public offering, converted more than 9 million shares to Class A from Class B, according to a document filed Aug. 10 with the U.S. Securities and Exchange Commission.

Thiel, a venture capitalist and hedge-fund manager whose proceeds from the IPO topped $630 million, still holds a stake worth about $550 million.

Ashley Zandy, a spokeswoman for Facebook, declined to comment.