A very interesting and convincing argument:
http://www.knewton.com/blog/knewton/from-jose/2011/06/03/groupon-is-a-straight-up-ponzi-scheme/
“The model is only sustainable if it pays off for local merchants â and to justify Grouponâs current size, it now must pay off for local merchants ubiquitously and flamboyantly. If not, Groupon is mostly a Ponzi scheme.”
….