The large user base
Half a billion and growing, is a really big number. It can quickly have people adopting it and there’s a very good chance that they won’t change their service provider (Facebook) if the service is decent enough.
One gets to see the deals, see what their friends have chosen, make their choice, check-in and post their updates to their timeline; all without leaving the Facebook interface.
Ability to pay through the platform
This one’s a clincher. Facebook already has Facebook credits which are used as the de-facto currency in social games and virtual trade on the platform. Why not extend it to the ability to buy real goods and pay for services.