not worth it
Groupon is definitely big but it's definitely not worth 5-6 billion. They made a good decision buying youtube because it had high and solid traffic with flexible technology and could integrate with what they already had.
Groupon seems to be one of those trends on the internet like myspace and twitter. It will definately have a large userbase but I feel that people just flock to these sites because it's the "new thing". People don't go on to these sites for specific products. It's always the odd products and services.
Reports About Groupon's Revenues And Profits Don't Make Sense -- Here's Why
They also take a 30% cut from sales which is a lot with huge competition in an industry where others can steal their business by simply offering to take a smaller piece of the pie (a lot like aff networks). On top of that users don't really have an incentive to stay with just groupon so users can easily switch to a better discount site.
I don't see why google can't make their own groupon... They have a missive brand. Have already reached tons of local businesses. Can then leverage the deals on adwords. Fucking recipe for massive benjamins!