Whilst I don't think a profitable business like Groupon will 'burst', I think it will slowly deflate. Why? Does anyone remember the company from over a decade ago, backed by Paul Allen, who were going to revolutionise ecommerce by offering group discounts? It was Mercata and they went under in 2001.

The article at http://www.seattlepi.com/business/vc122.shtml sums up why long-term, Groupon will face challenges (and it's also the exact reason why I decline to use Groupons' services):

"Those who wanted a baby jogger or DVD player, for example, sometimes had to wait seven days for Mercata's so-called PowerBuy to kick in. By that time, shoppers had moved on to another store or online shopping site. Instead of creating a faster, cheaper way to buy products, Mercata actually slowed the buying process."

Que Groupon offering group-buying service with no time-limits, only a quantity-limit?