Bitfinex is now the largest USD exchange and has processed 15M$ in the last 24 hours. They charge something like the standard 0.20%, that means they are pulling 20M$ annualized revenue, of which probably 50% are profits. Now, I remember using Bitfinex a year ago and it was full of bugs. The operator seemed to be a complete amateur (at least he did not lose any clients funds so far). Why is this exchange so large? Because of margin trading? It boggles my mind how little exchange innovate and how much money they pull in.
They have kept innovating vs someone like bitstamp who haven't improved 1 bit and also are the most expensive site to trade on.