Facebook chief executive Mark Zuckerberg’s personal wealth has taken a hit, knocking him off the Bloomberg Billionaires Index, which ranks the top 40 billionaires in the world.

Since his company went public on May 18, the social network co-founder’s personal wealth on paper has dropped to $14.7 billion from $16.2 billion, Bloomberg reported after the company’s shares fell nearly 10 percent in Tuesday trading. The stock closed Tuesday at $28.84, and was trading slightly up Wednesday morning at $29.33.

It was the first time that Facebook shares had fallen below $30. The initial public offering earlier this month, one of the largest in U.S. history, is now also one of the worst-performing, having lost over 20 percent of its value in seven trading days.

Zuckerberg’s wealth is now $800 million below the last person on the list, Colombian banker Luis Carlos Sarmiento.

Trading at $29 per share, Facebook’s price-earnings, or p/e, ratio is high, at about 62.9 — and investors don’t see justification for paying such a high price for such a small return.

According to Bloomberg’s data, the stock price would have to drop to $23.07 to have the same average p/e ratio as other technology companies on the Nasdaq index, based on estimated earnings in the next 12 months. For example, midday Tuesday, Google’s p/e was significantly lower at 17.7, and Apple’s was at 13.9.

Questions about Facebook’s revenue model have plagued the company, particularly as it audience shifts the way it views the site. The social network has made it clear that it’s trying to figure out how to secure its revenue stream over mobile devices.

Many analysts had said ahead of the initial offering that the stock was overvalued, though they believed that the stock would do well in its debut. But technological glitches delayed Facebook’s debut and forced the company’s underwriters to prop up the share price on its opening day; shares closed just 23 cents above issue price on the first day of trading. After that, Facebook stock never recovered.

Analysts have pointed to a number of other factors in the disappointing market performance. Shares are now trading in the original $28-$35 price range the company had set for its IPO, an indication that those handling the deal’s offering may have been too ambitious when they raised the price to $38 for the debut. Analysts also criticized the decision to release more shares to the public than originally planned, which lowered demand for the stock.

Zuckerberg, who has been away on his honeymoon after his surprise marriage to his long-term partner Priscilla Chan, hasn’t made a public comment on the stock’s performance since the company’s May 18 debut.

He has largely avoided media coverage of his honeymoon, though blogs made a lot of hay out of the fact that, as the Daily Telegraph reported, he didn’t tip the waitstaff following a lunch in Rome.

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