Facebook-owned Instagram has traditionally relied on Foursquare for its vast database of points of interest.
For example, upon uploading a photograph Instagram gives you an option of choosing a location where the image was taken using its Foursquare integration, simply by tapping a 'Name This Location' button.
Post acquisition, however, things have become a little weird.
Facebook has long been maintaining its own location database, Facebook Places. Two years into the acquisition, Instagram has now started using Facebook Places as its mapping service, at least for a subset of its users.
Despite this (expected) change, more than the 150 million Instagram fans can continue to share their check-ins to Foursquare from Instagram...