With the corporate earnings season drawing to an end and recent U.S. economic data raising doubts about the pace of growth, the S&P 500, which is down 7.3 percent so far in May, could decline further this week as concerns about the financial health of Europe persist.

"What has changed in the world since April? We went from hearing a constant refrain that the world is awash in money and markets must go higher to hearing nobody wants to take any risk. ... All in a week," said Peter Cecchini, global head of institutional equity derivatives at Cantor Fitzgerald & Co in New York.

The S&P 500 fell 4.3 percent last week, its steepest weekly decline this year, and closed below 1,300 for the first time in four months.

The hotly awaited market debut of Facebook on Friday was marred by technology glitches on the Nasdaq in sending messages back to the brokerages that handled orders of Facebook Inc for individual, or "retail," investors. Those problems rekindled fears about the market's electronic trading system and caused some investors to stay away from equities.

Weighing on sentiment is a growing sense among investors that the eurozone debt crisis is nearing new heights, fueled by fears of the potential for a Greek euro exit and the deteriorating health of the Spanish banking system

Solid corporate earnings and upbeat U.S. economic indicators had fueled the rally in U.S. stocks, offsetting jitters over Europe. But with earnings almost out of the way and data starting to disappoint, investors have shifted their focus back to headlines out of Europe.

Leaders of the Group of 8 major industrial economies met this weekend to try to tackle the financial crisis in Europe. U.S. President Barack Obama, the G-8 host, has urged European leaders repeatedly to do more to stimulate growth, fearing contagion from the euro crisis that could hurt the U.S. economy and his chances of re-election in November.

"The market is extremely oversold. Nonetheless, all major indicators remain on sell signals," said Larry McMillan, president of options research firm McMillan Analysis Corp, in a report on Friday.

"We expect a powerful but short-lived rally should be coming soon. But at this point, barring some major shifts in our indicators, it may only be a rally in a larger down-trending market," McMillan said.

THE FACEBOOK EFFECT

Facebook, the No. 1 online social network, disappointed investors with a tepid market debut on Friday. Shares rose a scant 0.6 percent — nowhere near expectations for double-digit gains on the first trading day — and the day was marred by technical problems due to huge order volume. The stock closed at $38.23 after falling as low as $38, its initial offer price.

The disappointing debut curbed investors' appetite for other social media stocks. Hardest hit was Zynga Inc., which closed down 13.4 percent to $7.16 after falling as low as $6.40. The stock was temporarily halted twice due to sudden declines.

LinkedIn shares fell 5.7 percent to $99.02, and Groupon fell 6.7 percent to $11.58. Zynga and Groupon, both of which went public late last year, are also trading below their IPO prices.

Despite the disappointing market debut and the weak performance of social media stocks, market participants are still optimistic about Facebook going forward.

"In any brand new area, social media in this case, most are going to be losers and only some are going to be winners. Yes, the IPO was disappointing, but Facebook is clearly the winner here and others aren't," said Randy Warren, chief investment strategist at Warren Financial Service.

This week's economic data includes April's existing home sales on Tuesday at 10 a.m. Existing home sales are forecast at a 4.60 million-unit annual, up from 4.48 million in March.

New homes sales figures are due on Wednesday at 10 a.m. April's new home sales are also expected to post an increase, gaining about 7,000 units over a 328,000-unit annual rate in March.

Initial jobless claims and durable goods orders will be published on Thursday at 8:30 a.m. Consumer sentiment is due at 9:55 a.m. on Friday.

For last week, the Dow fell 3.5 percent and the Nasdaq sank 5.3 percent.