If it's not trivially easy to make a profit on a company like groupon where they split revenue 50/50 with the companies they advertise for, there has to be something deeply wrong.
They may be splitting revenue 50/50 with their customers, but their customers frequently have to give up most of their profit or even take a loss to be able to do business with Groupon. In other words, their customers aren't exactly beating down their door; a significant, and expensive, sales effort is required.