Online Currency Exchange

Online portals are mostly taking less commission on money exchange as compared to other service providers. But there are a few who say that they are offering 0% commission. How come they earn then if there is nothing that they are taking?

The first 5 years that facebook was around, they didn't put any publicity on the site at all. Nothing.
Just increasing userbase. That was the goal. The money will come later.
Guess that's the case here too.