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Out of over 10,000 developers using Foursquare's API and location database, Instagram is the most frequent user.

Valentina Park
Valentina Park on July 12, 2011. @valentinajpark

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

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