It’s a very simple idea: users pick a political party they want to “vote” for with their check-ins. Every time you check-in, you cast a vote for your party, and GeoPollster tracks which party has the most votes in each state, venue, and category of venue. The app gives added weight to mayors and users with lots of badges.
While the site presents this as a serious polling resource, this is obviously for entertainment purposes only — the results tell us more about the politics of Foursquare/GeoPollster users than about the states and venues in question.
But if Foursquare, Facebook Places, and other location-based services ever achieve mainstream adoption, check-in analytics could become extremely valuable. The obvious applications have to do with marketing, but GeoPollster is a good example of how widely useful that sort of data could be.