Instagram first launched on iOS. Its initial app was built for Apple’s iPhone and became available through the App Store in October 2010, well before Instagram expanded to other mobile platforms.
The choice reflected the founders’ strategy. Kevin Systrom and Mike Krieger had originally worked on Burbn, a broader check-in application, but focused the project on simple mobile photo sharing. The iPhone 4’s improved camera also made it an attractive platform for a photo-centered service.
Android users had to wait until April 2012 for an official Instagram app. That release was a major expansion: it reportedly passed one million downloads in less than a day. BlackBerry OS and Windows Phone were not Instagram’s first launch platforms.
A common mix-up is treating the October 6, 2010 iOS appearance and the October 12 public Instagram release as exactly the same event. The earlier date involved a beta or testing version; the later date is commonly identified as the stable public launch under the Instagram name.