Investors quickly started selling the stock on the Nasdaq exchange Monday, sending the price of Facebook shares down 13 percent in the first hour. It later regained some strength, but still was down more than 10 percent in midafternoon trading at below $35 a share.

　　Facebook shares traded publicly for the first time on Friday, when they edged up to $38.23, 23 cents above the initial offering price.

　　With more than 900 million users, the social networking site is a well-known product. However, some financial analysts say they think the company is overvalued, and some projections of its future advertising revenue have been reduced.

　　The Nasdaq exchange said Sunday it mishandled the opening session of trading. Some investors say they lost money because of technical problems that prevented some large and small investors from completing stock trades. Nasdaq says it is recreating the mishandled trades to determine which stock buyers and sellers are owed compensation.