Out of over 10,000 developers using Foursquare’s API and location database, Instagram is the most frequent user.
According to a Foursquare spokesperson, the most popular third party app that uses Foursquare API for location-based check-ins is Instagram, followed by Tweetdeck, which was recently acquired by Twitter. Other top check-in apps include the likes of Sonar, Hootsuite, GetGlue. and ironically, one-time Foursquare rival Gowalla. GigaOm