The 28-year-old’s fortune plunged to $12 billion from $13.7 billion yesterday, according to the Bloomberg Billionaires Index. Shares of the world’s largest social-networking company sank as much as 17 percent after the company reported its second-quarter operating margin dropped from a year earlier and sales grew at the slowest pace on record. The shares were down 13.7 percent to $23.62 at 11:50 a.m. in New York.

“They are probably a little bummed out right now,” Herman Leung, an analyst at Susquehanna Financial Group, said in a telephone interview from his San Francisco office. “Facebook has to balance more of a fine line between delivering numbers for Wall Street and deciding how much monetization it’s going to push toward impacting its user base.”

The Menlo Park, California-based company was down 38 percent from its $38 offering price. Facebook raised $16 billion in its IPO, the largest ever for a technology firm. It is the worst-performing large initial public offering in the past decade.

Sandberg, Parker

Other Facebook shareholders have also taken a hit. Dustin Moskovitz, 28, who started the company with Zuckerberg from their dorm room at Harvard University, owns 133.7 million shares of the company’s Class B stock worth $3.2 billion, down $1.9 billion since the IPO.

Eduardo Saverin, 30, has a $1.3 billion stake, down $765 million since the offering. According to a regulatory filing dated May 17, he owns 53.1 million shares of the company.

Sean Parker owns 66 million Facebook shares valued at $1.6 billion. The 32-year-old persuaded Zuckerberg to move to California to focus on the company full-time in 2004.

Facebook’s chief operating officer, Sheryl Sandberg, 42, who was lured from Google in 2008, owns almost 16 million shares and vested restricted stock units, according to data compiled by Bloomberg. They are valued at $377 million, down $229 million since the IPO.

To contact the reporter on this story: David De Jong in New York at ddejong3@bloomberg.net