Groupon may never make money, but some company with a similar business model will eventually make a profit. What deal of the day sites offer that no one did before is an immediate and essentially limitless supply of customers.

Even if most companies lose their shirt on that, there are going to be some industries and situations where infinite customers at a temporary loss makes business sense, and once the market for it becomes rational again someone will be able to do it better than they could internally and make a profit on it.