According to Apple's latest numbers, iOS 9 adoption is moving along fairly quickly. Apple's latest mobile operating system is now in use by 57% of iOS device owners just three weeks after its initial launch on September 16, 2015. This data comes from the company's own App Store distribution numbers.
iOS 9's adoption rate compares quite favorably to last year's numbers for iOS 8, which at this point a year ago stood at 47% adoption. Currently, iOS 8 stands a 33%. Older versions of iOS, from iOS 7 on down, are installed on 10% of iOS devices.
There are a number of factors contributing to iOS 9's popularity. For one, the update was considerably smaller than previous updates, coming in at just over 1GB for most users. Additionally, the recent release of the iPhone 6s and iPhone 6s Plus, which only run iOS 9 and above, no doubt helped give it a boost. iOS 9 is also available to more devices than any previous update.