If you plotted out all the check-ins made on Foursquare, you should be able to get a pretty good handle on the geographic layout of a city. It's like modern day map making or check-in cartography. So do you think you can recognise a city just by its Foursquare check-ins?
Foursquare slapped together a map of the past three month of check-ins (that's 500,000,000 check-ins. Half a billion!) to show how Foursquare check-ins can show you the walkways of Central Park, the line of Golden Gate Bridge and more. Foursquare is able to see patterns in people's behaviour (if a lot of people go to a taqueria after watching a movie, it can recommend it) and have an ever-updating database on where people are going.
So, let's play a little game of guess the city. The one above is Istanbul. Pretty bright, huh! [Foursquare]
You sound like you're from London!
Foursquare sketches the island that is Oahu.