U.S. stocks dropped for a sixth day, the longest slump since November for the Standard & Poor’s 500 Index, as Facebook Inc.’s record initial public offering failed to boost confidence in a market rattled by Europe’s debt crisis.

Facebook rose 0.6 percent, paring a rally of 18 percent. Underwriters bought the stock to keep it from falling below the IPO price, people with knowledge of the matter said. The pricing of the first transaction took a half hour longer than Nasdaq OMX Group Inc.’s forecast. Zynga Inc. tumbled 13 percent to a record low after a volatility circuit breaker prompted two trading halts that lasted at least 10 times longer than usual.

The S&P 500 dropped 0.7 percent to 1,295.22 at 4 p.m. New York time and has slumped 4.6 percent in six days. The Dow Jones Industrial Average lost 73.11 points, or 0.6 percent, to 12,369.38. The Nasdaq Composite Index fell 1.2 percent to 2,778.79. About 8.8 billion shares changed hands on U.S. exchanges, the most since Dec. 16.

“There was way too much hype with Facebook,” said Philip Orlando, the New York-based chief equity strategist at Federated Investors Inc. His firm oversees about $370 billion. “If it had enjoyed a better move, there would have been some coattails for the broader market. There’s pessimism related to Greece, there’s a G-8 meeting and nobody knows what’s going to come out of it.”

Stocks fell a third week in the longest losing streak since August. German Finance Minister Wolfgang Schaeuble said turmoil in the financial markets caused by Europe’s crisis may last another two years, as Group of Eight leaders prepared to discuss Greece and its impact on the global economy. LCH Clearnet Ltd., Europe’s biggest clearing house, raised the extra deposit it demands from clients to trade some Spanish government bonds.

Equities rallied earlier today amid optimism about Facebook’s trading debut. The delayed opening and halts in other companies including Zynga weighed on the market following the year’s most anticipated IPO.

Nasdaq OMX said in a statement posted to its website at 11:59 a.m. New York time that it was having a problem delivering trade confirmations related to the IPO. “Nasdaq is working to deliver these executions back to customers as soon as possible,” according to the notice. The company said in a message at about 1:57 p.m. that the delayed messages had been sent. Rob Madden, a Nasdaq spokesman, didn’t respond to requests for comment.

Nasdaq OMX shares slumped 4.4 percent, the most since October, to $21.99.l

Facebook advanced 0.6 percent to $38.23. Its IPO price of $38 valued the company at $104.2 billion, or 107 times trailing 12-month earnings, more than every S&P 500 member except Amazon.com Inc. and Equity Residential.

Zynga tumbled 13 percent to $7.16. The shares paused for a volatility circuit breaker that normally lasts five minutes, and failed to resume until about 50 minutes later. Another halt lasted more than an hour.

The stock was paused at 11:37 a.m. New York time after dropping as much as 14 percent from yesterday’s close to $7.08. The circuit breaker is triggered when a company rises or falls 10 percent within five minutes. It resumed at 12:27 p.m. and was halted again two minutes later because of volatility. Trades commenced at 1:35 p.m.

A measure of technology shares in the S&P 500 fell 1.2 percent for the biggest decline among 10 industries. Apple Inc., the most valuable company, gained 0.1 percent to $530.38, snapping a five-day tumble.

Autodesk Inc. tumbled 13 percent, the most in the S&P 500, to $30.26. The maker of architectural and engineering software provided a second-quarter earnings forecast that trailed some analysts’ estimates.

Yahoo! Inc. rallied 3.7 percent to $15.42. The company is in talks to sell about 20 percent of Alibaba Group Holding Ltd. back to the Chinese Internet company for about $7 billion, according to a person with knowledge of the matter.

Salesforce.com Inc. advanced 8.8 percent to $145.58 for the biggest gain in the S&P 500. The largest seller of online customer-management software forecast fiscal second-quarter sales that beat estimates as it signs more large deals with corporate customers.

The slump in U.S. stocks has pushed a gauge of market momentum to levels last seen in August, a sign to some traders that the S&P 500 fell too far, too fast.

The benchmark gauge for U.S. equities dropped 8.1 percent through yesterday from a four-year high on April 2, sending the 14-day relative strength index for the S&P 500 to 25.1, the lowest level in nine months. The indicator fell to 23.22 today.

The RSI, which measures the degree to which gains and losses outpace each other, has been below 30 for three consecutive days. Readings that low are considered signs to buy by some analysts who use charts to make forecasts.

“RSI is like a rubber band,” Josh Dollinger, chief quantitative and technical strategist at BTIG LLC in New York, wrote in an e-mail. “The further back it’s pulled, the faster and harder the snap back is. It’s difficult to time perfectly but over the long term, buyers are often rewarded when stepping in during these rare, extreme circumstances.”

The momentum indicator sank to 16.5 on Aug. 8 after S&P cut the U.S. government’s credit rating from AAA. The S&P 500 then slumped 1.8 percent through Oct. 3 before surging 29 percent by April 2.

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