The social component, making user-to-user recommendations part of the coupon/deal experience.
Admittedly though, I find some things go slow with Facebook. E.g. Facebook’s Places/Check-ins are not nearly as dynamic and “lively” as Foursquare and Gowalla, and I still see most of my friends checking in at those other services and then putting an update about it on Facebook. Why is this? Better mobile apps? Better features? Boring displays of Places (a fact)?I don’t know.
Will the same happen to Deals, or is it just a matter of Facebook being a late comer, but due to its massive user base will dominate over time?