If they are pulling in $2billion a year as the stories are saying, $6B is nearly chump change. Plus it keeps the humans involved in Groupon rather than Google trying to replace it with algorithms.
http://online.wsj.com/article/SB1000142405274870467920457564...
"Currently about half of Groupon's 3,000 employees are in sales"
That explains a lot. They have a ton of telemarketers pushing onto every business with a phone number. This reminds me a lot of the "local directory" space (which the WSJ article claims they're occupying, but from the POV of the small business owner and the consumer it looks like a very different product). There was a lot of excitement and initial signups when things first got off the ground (just a bit before the bubble burst), but the churn can be horrendous.
The difference with Groupon is that the "ads" are actually accountable and produce sales compared to local directory ones. But there's been a few horror stories about dumb business owners getting fucked over with "we're losing $1 on every sale, but we'll make up for it on volume!" deals. Their own fault, but I bet the Groupon telemarketers are pushing it on them like Walmart pushes on suppliers.
What I'd like to see is the current churn rate. If Groupon and competitors keep going at the current pace and businesses keep getting burned, eventually they'll burn the market.