Two that come to mind are Coinbase & Circle. I did a quick google search, possibly wrong terms. The information I got was useless. I am sure this has been discussed. If there is a good article/link on this it would be appreciated.
Fee for selling and buying and enlisting new coins to their exchange.For example purchasing bitcoin with credit/debit card,coinbase charge up to 3.99% service fee.Circle has different fee structure.I have seen some exchanges even have earnings from ads
Not much exchanges have ads on their websites but only those exchanges have ads which will be in the early stage and which will not have much coins to trade and which are unable to get any higher fees from the transactions because of their early stage.