See and compare mobile platforms used by your Foursquare friends.
Plat4Mobile - Mobile Platforms on Foursquare
This app si limited to your last 100 checked-in friends (this is limitation given by Foursquare API). In MOST CASES it covers ALL your ACTIVE friends on Foursquare. But if you have more than 100 active friends, the statistics could slightly differ from day to day (depends on which of your friends have been just checked-in).