Since launching in September alongside the iPhone 6s and 6s Plus, iOS 9 has seen its adoption rate grow to 66 percent of active iOS devices, according to numbers on the Apple Developer Support page. Two weeks ago, iOS 9 adoption sat at 61 percent.

After 24 hours, iOS 9 adoption was in line with the adoption rate of iOS 8. However, in late September Apple announced that the newest version of iOS had the fastest adoption ever and was on pace to be downloaded by more users than any other version of the mobile operating system. iOS 8 only achieved a 64 percent adoption rate by December 23 and a 68 percent adoption rate by January 7, ending up with a high of 87 percent on the eve of iOS 9's launch.

Apple released the first major update to iOS 9 two weeks ago with iOS 9.1, which featured a slew of brand-new emojis. Since that update, iOS 9 adoption has grown by 5 percent. Earlier today Apple seeded the second beta for iOS 9.2, the second major update to its newest mobile operating system.