Just four days after its release on September 17, iOS 8's adoption rate hit 46%, according to Apple's App Store Distribution page. This is well ahead of iOS 7's rate, which hit 52% a week after it was made available.
At the end of May, just three days before Apple announced iOS 8, around 90% of iPhone users and 85% of iPad users had adopted iOS 7, which means iOS 7 has dropped of somewhere in the neighborhood of 40% in just a few days.
Aside from multitudes of people upgrading their iPhone 5's and iPads and older devices, many millions of people received and activated their new iPhone 6's two days later, meaning that millions of iOS 7 devices were deactivated. That number continues to grow as demand for the new devices remains at a level that Foxconn is struggling to meet.
iOS 8 adoption has likely surpassed iOS 7's 52% one week benchmark by today, which marks one week since its release. Also marking the one week anniversary, Apple today released iOS 8.0.1 which contains some bug fixes.