Everything they have done so far indicates great competence in what they are doing. There are numerous companies that have blown hundreds of millions of dollars on advertising and ended up with a tiny user base (I can think of a particular MVNO a few years back.) Then you have failed acquisitions such as Cisco buying Flip, where the buyer ends up shutting the company down.
Neither Groupon's customer or business acquisition strategy is a coin flip. They know very well what these users and business's are worth and they have all of the tools in place to monetize them.
Without an extremely aggressive and hyper growth strategy, Groupon stands little chance against Google or Facebook. If they wanted a path on easy street, the original investors could have sat around pocketing millions of dollars a month for years. Once Groupon decided to go big or go bust their profile was raised to the point that ceased to be an option.
The real wild cards here are not so much what Google and Facebook do but rather what the business clients do. There is a very fine balance between delivering great value to both sides of the equation. Tipping too far in one direction destroys their brand. I've heard plenty of complaints from users about the quality of Groupon's deal offerings.
If Groupon can continue to demonstrate an ability to successfully overcome these challenges while staying a step ahead of Google and Facebook, they might be around for a while.
Will they deliver fantastic returns for shareholders at a mature $20 billion valuation over the long term? That is a completely different question, but if you think that an answer of no equates Groupon with a ponzi scheme, all financial markets are ponzi schemes.