Since I've been in New York, I've started to explore Foursquare. And although I'm finding the whole checking-in thing tedious, I'm excited at the prospect of using the type of data that Foursquare is creating and applications for its API.
You'll see below an interesting use of Foursquare's API for the recent US mid-term elections. Foursquare users checked-in, and then they unlocked a 'I voted' badge that was specially created for the election. Only 50,000 people did it, but there is something very, very interesting about the idea. Driving hyper local advocacy is Foursquare's strength. The phrase, "All politics is local" is a common phrase in U.S. politics. The former U.S. Speaker of the House Tip O'Neill coined this phrase which encapsulates the principle that a politician's success is directly tied to his ability to understand and influence the issues of his constituents (Wikipedia).
Its not brilliantly executed, and the search function doesn't seem to work, but you can see how politicians might be able to use this going forward. You drag around the map to navigate. More information on Foursquare and election app here.