Brendan McDermid/Reuters

Shares of Facebook were down 11 percent in trading on Monday, well below the company’s offering price and again raising questions about its lofty valuation.

In its second day as a public company, Facebook closed at $34.03. That extended the trading patterns from late on Friday, when shares repeatedly bumped against their $38 offering price.

On Friday, traders at the company’s lead underwriter, Morgan Stanley, resorted to buying shares to prop up the price at $38, an important psychological barrier. To break the offering price would lead to what traders call a “busted I.P.O.,” an embarrassing moment for the underwriters.

It was unclear on Monday morning whether Morgan Stanley had again stepped in.

Technical problems at the Nasdaq stock market on Friday halted trading in Facebook for a half hour, and led to investor uncertainty about whether trade orders had gone through.

Nasdaq’s chief executive, Robert Greifeld, said he was “humbly embarrassed” by the technical missteps, brought on by an enormous volume of trading. The exchange has since announced changes to how it will handle initial public offerings in an effort to avoid problems in the future.

But Mr. Greifeld said the firm’s own data showed that the Friday morning delay had no effect on the opening price, and that Facebook’s trading had correlated with other stocks in the market.

“It would lead a reasonable person to conclude that it didn’t have an impact on the stock price,” he said.

The sell-off is sure to leave analysts and shareholders wondering whether Facebook was worth the enormous $105 billion market capitalization it gained from its initial public offering. Critics have said the company does not generate nearly enough advertising revenue to justify being priced at 108 times earnings over the last 12 months.

The broader markets appeared to be doing better. The Dow Jones industrial average, the Standard & Poor’s 500-stock index and Nasdaq were all up midday.