“location based” social networking or as Foursquare puts it “See where your friends are, learn about where they frequent and unlock rewards as you travel about the city“. That last bit (my italics) gives away the revenue model- i can’t think of another social media brand that is so nakedly commercial in its mission.
“Right Person, Right Place, Right Time” is the holy grail of sales response marketing- the categories that Foursquare want to take income from are neatly shown in this screen grab on their demo film. Expect all social networking services to beef up their location based stuff more.

