If Facebook buy Groupon, then Facebook can make Groupon growing at a fastest page with over 900 million users all around the world. Groupon will be a giant competing with Amazon and Priceline. With 900 million users from Facebook, Groupon can cut off all of its sale people, and turn it to profit for the company.
I think there was a reason that Google offered $6 Billion bid last year when Groupon making only $300 million in Revenue, now it's revenue jump to $1.6 Billion. So Groupon valuation should be around $12-$15 Billion, imo. Priceline.com could jump in to outbid Facebook.
In the future, when Groupon has enough customer base, then it will cut off marketing cost and create an auto pilot to add customer base.
Groupon is a cashcow now. Therefore Groupon stock will not be affected by the lockup expiration.