The a16z Marketplace 100

In comparing the two categories, our theory is simple: There's a big difference between network effects that span global and continental regions, as opposed to city-by-city networks. Airbnb's global network effect spans regions, since potential guests book lodging outside their home base and hosts expect to receive visitors from all over the world […] City-by-city network effects are naturally more fragmented and suffer from more severe competition. In other words, winner-take-all-dynamics are nonexistent.

A really great insight into why there have been so many local buisness customer loyalty products launching recently in NYC.