Nearly one out of every five iPhone users are now running iOS 9 on their devices, making it the second most popular version of iOS after iOS 8.

Just less than 48 hours after Apple released the newest version of its mobile operating system on Wednesday, Sept.16. nearly 20 percent of all iPhone users in the United States have already downloaded and installed the version into their devices, despite early hiccups that prevented many users from downloading the OS the minute it launched. 

Mobile analytics firm Mixpanel reports, as of early Friday morning, Sept.18, the uptake for iOS 9 has soared to 18.8 percent, up from the 12.6 percent adoption rate 24 hours after the software's release. By comparison, iOS 7 and older versions comprise only 9.2 percent, while iOS 8 remains the most popular with 71.99 percent. Although adoption rate is not as fast as the iOS 7, iOS 9's numbers are expected to rise further in the coming days, considering iOS 8 peaked at 90 percent.

"One huge advantage Apple got right this year over last year was that the update is much smaller than last year (3.5x smaller)-Apple has empathy for its users who have nearly maxed out their space on their phones," says Mixpanel CEO Suhail Doshi. "However, one issue for it being substantially slow compared to iOS 7 is that servers are having trouble keeping up right now as people update."

Mixpanel's numbers are based on data collected from thousands of mobile apps it collects by determining the devices from which these apps are accessed. The method is not entirely accurate, and similar analytics firms who employ the same method but monitor different apps may come up with very different results.

For example, Tapjoy, which claims it has a reach of 200 million iOS users each month, says adoption rate for the iOS 9 on the iPhone is seven percent, while uptake for other devices is even lower. Fiksu's Jeremy Sacco says the firm sees six percent. Meanwhile, GoSquared's numbers more closely align with Mixpanel, as it says adoption rate for the iOS 9 as of Friday is 14.7 percent, while iOS 8 is at 70.66 percent.

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