NEW YORK (Reuters) - Stocks fell at the open on Wednesday on mounting concerns over Greece's future in the euro zone and as a weaker-than-expected revenue forecast from computer maker Dell weighed on investor sentiment. The Dow Jones industrial average dropped 64.41 points, or 0.52 percent, to 12,438.40. The S&P 500 Index fell 7.93 points, or 0.60 percent, to 1,308.70. The Nasdaq Composite Index lost 24.59 points, or 0.87 percent, to 2,814.49. (Reporting by Angela Moon, editing by Dave Zimmerman)

(Reuters) - Facebook Inc and banks including Morgan Stanley were sued by the social networking leader's shareholders, who claimed the defendants hid Facebook's weakened growth forecasts ahead of its $16 billion initial public offering. The defendants, who also include Facebook Chief Executive Officer Mark Zuckerberg, were accused of concealing from investors during the IPO marketing process "a severe and pronounced reduction" in revenue growth forecasts, resulting from increased use of its app or website through mobile devices. Facebook went public last week. The lawsuit was filed in U.S. ...

BERLIN (Reuters) - The impact of a Greek exit from the euro zone would be substantial but "manageable", Germany's Bundesbank said on Wednesday, raising pressure on Athens to keep its painful economic reforms on track. In a toughly worded monthly report, the German central bank also said euro zone member states should have a say on further payments of aid to Greece under its 130 billion euro bailout program funded by the IMF and the European Union. ...

BRUSSELS (Reuters) - Euro zone officials have told members of the currency area to prepare contingency plans in case Greece decides to quit the bloc, an eventuality which Germany's central bank said would be "manageable". Three officials told Reuters that the instruction was agreed on Monday by a teleconference of the Eurogroup Working Group (EWG) - experts who work on behalf of the bloc's finance ministers. ...

SAN FRANCISCO (Reuters) - With two of its most distracting conflicts resolved in the past week and a half, Yahoo Inc hopes its new interim chief executive can focus on the biggest challenge of all: turning the company into an entertainment and information destination that wins back the advertising dollars flowing elsewhere. While ousted predecessors Scott Thompson and Carol Bartz were seen as technologists, Ross Levinsohn has managed Internet efforts at such mainstream media companies as CBS and Rupert Murdoch's News Corp. ...