Two reasons why Groupon can succeed as a long lasting business.
Adam Rifkin stashed this in Groupon
Stashed in: Silicon Valley!
They hire great executives: http://techcrunch.com/2012/04/13/in-defense-of-groupon/
Plus, they enjoy network effects. None of Facebook, Google, and Amazon have made progress building a Groupon killer.
Either Groupon is more defensible than it seems thanks to great management and network effects, or this business really isn't worth pursuing full force, which could be why competitors have backed off.
Time will tell.