While iOS 10 initially fell behind previous OS versions in terms of adoption rate, the new iteration of Apple's mobile software is now installed on two-thirds of iPhones and iPads, based on the latest market analyses.

The data is particularly useful for app developers to gauge how fast the market, which is using the new OS, is growing and assess the next few updates to their own apps.

In early October, third-party software and app tracking groups Mixpanel and AppLovin placed the figure north of 50 percent two weeks after the new iOS was released. Since Sunday, however, Mixpanel reported 66 percent of iDevices have started operating on iOS 10.

For mobile marketing technology analyst Fiksu, the number is slightly higher at record-breaking 66.7 percent. The rate shows at this point the fastest adoption of any iOS.

These numbers indicate that two out of every three active iPhones and iPads now rely on iOS 10. The spike in usage is considered unusual because adoption of iOS 10 picked up remarkably only between Sept. 27 and Oct. 4, when it registered a 20 percent increase in this short period, according to Fiksu.

iOS 10 Wasn't An Early Hit

On Sept. 5, about a week before the launch of iOS 10, Mixpanel recorded that 89.31 percent of active iDevices were running on iOS 9. It dropped to 71.61 percent a week later.

But iOS 10 wasn't always readily embraced by users. In fact, it trailed behind the adoption rates seen with iOS 6, 7 and 9 during the iOS 10's first two weeks of life.

On Day 20 from the time of release, at 58.3 percent, iOS 10 began to slightly overtake the adoption rates registered by iOS 9 (57.7 percent) and iOS 6 (58 percent) within the same period after their own launch. But iOS 10 was still no match for the level of adoption received by the iOS 7 almost three weeks after the 2013 firmware went live.

Fiksu: 'Improved User Confidence'

Until iOS 10 pivoted to suddenly win the hearts of more iDevice users, the latest firmware was actually expected to perform subpar its siblings.

What caused the sudden jump is still unclear. Sale of the new iPhone 7 and iPhone 7 Plus would have likely bolstered the rate of adoption. But the initially slower pace of upgrade may have also been due to the advice most mobile OS experts give the public: it's actually best to wait a couple of weeks or so for possible glitches to be fixed first before jumping into a new OS.

"After a lackluster first two weeks, a combination of increased emphasis by Apple and improved user confidence in iOS 10 is pushing adoption rates to new heights," says Tom Cummings, Fiksu's VP for New Market Strategy.

Often, the brave souls who sign up early are the ones who eagerly await an update to the OS — bugs and all — and who are vocal about the early glitches that need fixing fast.

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