Pop over to CoinMarketCap and check Bitfinex's page.
You'll see that the trading pair with the second-highest volume on the exchange over the past 24 hours – nearly $48 million at the time of writing – is USDT/USD: the tether stablecoin, which aims for parity with the U.S. dollar, and the U.S. dollar itself.
From this data point, you could be forgiven for concluding that Bitfinex's customers trade huge quantities of tethers for dollars, and vice-versa – more than they trade ether, XRP and EOS combined for dollars.