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We already knew that Yelp would be integrated into a number of iOS features when iOS 6 was announced: Namely, that users would be able to access Yelp reviews using Apple's voice-command system, Siri.
Still, Apple's decision to integrate with Yelp over Facebook or Foursquare is surprising to some, given that: a) the latter two services are much better recognized and b) Apple is also introducing deeper integration with Facebook in iOS 6. It poses a particular threat to Foursquare, the bulk of whose users employ its iPhone app to check in and keep track of their friends' movements. If users can check in directly from the Maps application, they may no longer bother to open up Foursquare's app to check in through that network as well.