The main way facebook could challenge groupon+ living social 1) beyond groupon and living social, there are lots of deal-buying websites including gilt city, buywithme.com, etc. Because of the low multi-homing costs, facebook needs to be different from just providing deals. It has to successful make deals a SOCIAL EXPERIENCE with simple tools to invite friends to join deals, check who bought the deal, etc.
Facebook's deep penetration of the social graph combined with their uncontested dominance of highly specific targeting information definitely makes them a force to be reckoned with. They need someone that can execute though. Would be interesting to see them swipe a few of the top guys at either of those places.