The geography of Foursquare AUG 30 2011
Great annotated list by Dennis Crowley of places that contributed to the creation of Foursquare.
Foursquare (and it's predecessor, dodgeball.com) were designed and built in downtown NYC. Here's a walking tour of where a lot of the ideas came from.
As Steven Johnson said, this is a "case study in how urban space fosters innovation".