One concern comes to mind with an application like StreetSpark. By making your services a smartphone-based platform, you really close the market for your product. There are only a certain number of people in any given area, but by filtering those results with people who have a smartphone (iPhone specifically) AND are using your application can greatly reduce user inventory. Nonetheless, the idea of using the social graph to find matches is a great idea. I think StreetSpark would do better focusing on a web based platform with geo-dating as a supplemental feature.
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