Even though Groupon's share price has been sliding & has slid 50% since first going public its still north of $8.5billion so when Google got their calculators out and came up with $6billion they weren't too far wrong. There was a massive hype on GROUPON at the launch (and its since gone flat/south in price).
The logic here is that no one wanted to buy TZOO without the chance of control. Now that some peeps have figured out there's a deal to be had, perchance it will flush out other bidders. TZOO is currently $430million market cap. So if we got $ a shade under $1billion + then allowed a $1billion to increase/improve the company you've got Groupon 's brother for $2billion + $4billion still in the bank?
Zuckerberg needs good ways of generating cash to keep his valuations high. While TZOO hasn't got the hundreds of millions of subscribers that FB has, they are both programming companies and it doesn't take a genius to see the benefits that small scale working model provided by TZOO could be an excellent blend into the family of Facebook companies.
The reason Bartels has put himself up for sale, is IMHO that he realizes he's boxed in (relatively speaking) that the bigger players have so much deeper pockets that being acquired now before FB goes public could be so much better for his bottom line (& perchance the longs bottom line) is why this deal is going to go thru. After all said and done TZOO have hired financial advisors!