As of now, none of the five largest, most influential Bitcoin exchanges are in the United States: Slovenia-based Bitstamp ($368 million in 30 days); Bulgaria-based BTCe ($267 million in 30 days); Japan-based Mt. Gox for dollar transactions ($261 million in 30 days); China-based BTC China ($151 million in 30 days); Japan-based Mt. Gox for euro transactions ($31 million in 30 days). [Source]