Felix Salmon explains it:
[R]estaurants are much like Groupon itself. It makes money on every sale — but it will only see its margins start to rise impressively if and when its merchants start coming back on a regular basis. At that point, the cost of setting up and selling a new deal comes down dramatically, and Groupon’s profits on the deal — after accounting for the cost of their salesperson’s time — rise substantially. Groupon itself, as much as its merchants, is counting on repeat business. And that comes from having a positive reputation which can spread like wildfire over Facebook and other social networks.
The Dish 