It has < 50k downloads since its launch (almost half a year ago now), I wouldn't call it fantastic if the main function is to spread the word. (not that it is not a great product, the app is awesome!)

You don't need the app to use the service (cc: cash@square.com or request@square.com). We have no idea how many people are using the service.

Remember that for every single person that uses it, they gain another user on top of that. Even more potentially with request@. I consider that a fantastic marketing tool since it must effectively cost them very little to run the service.