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NEW YORK (CNNMoney) -- Stocks closed out an ugly week. Despite initial euphoria surrounding Facebook's public debut, the social network's shares barely popped above its offering price and failed to inspire investors to buy into the broader market.

All three indexes clocked their worst weekly losses of the year, finishing at the lowest levels since January.

U.S. investors focused on the global issues plaguing world markets Friday, which pushed stocks down for the third straight week.

On Friday, the Dow Jones industrial average (INDU) lost 73 points, or 0.6%. The index is down 3.5% for the week.

The S&P 500 (SPX) slipped 10 points, or 0.7%, and 4.3% for the week. The Nasdaq (COMP) fell 35 points, or 1.2% Friday and 5.3% for the week.

Facebook (FB), which priced its initial public offering at $38 a share after the closing bell Thursday, jumped 11% when it started trading mid-morning Friday, but closed just above break even line.

"People are talking about Facebook, but it's really a sideshow," said Win Thin, an emerging market strategist for Brown Brothers Harriman. "If Europe blows up, people will trade on that more than anything else."

The European debt crisis loomed over global markets. Asian stocks sold off sharply, based partly on the slowdown in the Chinese economy. European markets were also under pressure, and borrowing costs for Spanish and Greek debt remain high.

Concerns are mounting about a potential Greek exit from the euro, and the implications that it could have for other fiscally troubled nations such as Spain and Italy. Rating agency Moody's downgraded 16 Spanish banks Thursday, including giants Banco Santander (STD) and BBVA (BBVA), the latest sign of distress in Europe.

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Stocks: Worst week of the year