Swiss lender, UBS lost 349m Swiss francs ($356m) by investing in Facebook shares, more than halving its profits.
UBS's second quarter profits were $425m Swiss francs ($434m; £276m), compared with 1bn francs a year earlier.
Facebook was valued at $104bn at its flotation in May, but the shares are now 39% below the initial sale price.
As a result, UBS's investment bank reported a loss in the quarter, compared with a profit of 730m Swiss francs a year earlier.
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