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Facebook stock ended trading Monday 9.1%lower at $20.79 a share, as investors reacted to reports of a securitybug and renewed their worries about the company's future.

Shares initially suffered Monday following a weekend report in Barron's, the weekly newspaper published by The Wall Street Journal, claiming shares of the No. 1 social networking stock are overvalued and could fall to $15 a share.

Laterin the day, there was discussion on online news sites, alleging privatemessages were appearing online on users' public 'wall' pages. At onepoint, shares were trading 10% lower for the day.

"Asmall number of users raised concerns after what they mistakenlybelieved to be private messages appeared on their Timeline," a Facebookrepresentative said in an emailed statement. "Our engineers investigatedthese reports and found that the messages were older wall posts thathad always been visible on the users' profile pages. Facebook issatisfied that there has been no breach of user privacy," the emailedstatement said.

The stock was supposed to get aboost Monday, after being added to the obscure Nasdaq Q-50 index. TheQ-50 index is a list of the stocks likely to be added to the widelywatched Nasdaq 100 index, a collection of the 100 most valuablenon-financial stocks trading on the Nasdaq.

However,bears on the stock continue to say that Facebook is actually worth $15 ashare or less. The stock got a lift last week on the coattails ofpositive comment from CEO Mark Zuckerberg earlier in the month.

Shares,though, have been a disaster for investors who piled into the initialpublic offering that debuted May 18. Shares are now about 45% below theIPO price of $38 a share.