Swiss bank UBS lost 349 million Swiss francs ($US356 million) due to problems with the launch of Facebook shares.
UBS blamed the Nasdaq exchange's handling of the flotation and said it would be seeking compensation.
It said system problems had left it with more Facebook shares - which have fallen in value- than it had ordered.
UBS's second quarter profits were $425m Swiss francs ($US434 million), compared with 1 billion francs a year earlier.
UBS shares were trading 6% lower at 10.26 francs on Tuesday afternoon.
Facebook was valued at $104 billion at its flotation in May, but the shares are now 39% below the initial sale price.