I'll confirm the sales model, my mother runs an (awesome) coffee shop in Abbotsford, British Columbia, "Chapleos" - and she routinely gets a call from her Groupon Rep. I suspect that their direct sales staff are incented to contact small business, and update the database on them. To some degree, the principal assets of Groupon are (A) their Brand, (B) their Mailing list, (C) their direct sales force, and (D) That database of business which will soon, if not already, be the finest database of businesses in the world.

Both Yelp/Google have an issue with their data freshness on business. I'm constantly updating open/close times, (and even whether a business exists) on Yelp. And business (particular taxicabs) have long ago figured out how to Game Googles Small Business directory to get in fake listings.

It will be interesting to see what they do with their database.