Also - won't Facebook Deals serve as a formidable competitor to Groupon?
^^This.
The people who support Groupon walking away will cite that Groupon stands to earn $2BB this year, so a $6BB buy out only represents 3 years earnings.
On the other hand, Groupon's ability to sustain that is questionable. There are several academic articles and many anecdotal reports out there that support Groupon primarily being a "deal hawk" site and not actually resulting in repeat or a long-term increase in business for the small businesses that use the service.
Google was looking for a turn-key local solution. Facebook "deals" would represent a serious threat to Google's local user reach, and they've been stepping up their game in the local space with new maps roll-outs, Google places, etc. all year. Groupon would have been a quick, easy, already branded solution to bring under their umbrella.
That said- Facebook & Google can replicate and scale Groupon's biz model a LOT faster than Groupon can rival their userbase. (Yes, point out the failure of "buzz" as a twitter killer here- but you can't argue the fact that Groupon will never have the mass reach Google or Facebook does).
Groupon makes about $200k per employee.
Google, on the other hand, makes 1.2M per.
Groupon's founders just shot themselves in the foot. They could have walked away with 6BB now and moved on to another start-up or never worked again.