Amazon: Ponzi Scheme or Wal-Mart of the Web? (2000)
"...when you consider that better than 60 percent of Amazon's sales come from repeat customers--which implies that they're loyal..."That seems to be the key figure missing from the Groupon discussion.  If they are buying loyal users and customers, their mad-dash growth strategy would seem rational.  If their retention is poor, it would seem more like a Ponzi scheme.