According to some estimates, over 250 million iOS devices already have upgraded to Apple’s radically redesigned mobile operating system in four weeks.

iOS 7 ramped up very quickly once it was released September 18, according to data tracked by Millennial Media, http://www.millennialmedia.com which sells a mobile advertising and data platform called MMX Ad Exchange.

“After the first week, iOS 7 grew from having 0.5 percent of our total iOS traffic, to 16 percent of total iOS traffic,” wrote Matt Gillis, Millenial Media’s executive vice president, global product and platform, in a blog-post this week. It took less than four days for iOS 7 to surpass iOS 6 in their tracking. But by the end of week one, the rate of adoption slowed.

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“While the 185 percent daily growth for iOS 7 was impressive, we looked back at the first week of growth for iOS 6 and saw that it grew an average of 264 percent daily,” Gillis wrote. “Additionally, while the release day impressions for iOS 6 and iOS 7 were similar, once the release was seven days out, iOS 6 saw significantly more adoption, and was at 35 percent of our total iOS impressions, compared to the 16 percent for iOS 7.”

             Credit: Millennial Media

Mix panel data shows a similar pattern of iOS 7 growth, with very strong adoption in the first week. By Oct. 1, iOS 7 penetration was at 65 percent. But from Oct. 1 to Oct. 18, the penetration rose much more slowly, to just over 72 percent.

The Android universe is much less uniform, according to Mix panel. This chart shows the relative penetration, to date, of the various Android versions. As noted about 45 percent of this sample is using some form of Android 4.x, or Jelly Bean. Less than a third, or 28 percent run 4.1.2. Less than 10 percent are running one of the three most recent 4.2.x releases.

But this progress, according to John Koetsier in a Sept. 4 post at Venture Beat. “Just days after announcing the next version of Android, Kit Kat, and telling the world that a billion Android devices have been activated, Google updated what used to be Android’s dirty little secret: fragmentation between various versions of the mobile operating system,” Koetsier wrote. “And Android has never looked more unified.”

The somewhat slower growth of iOS 7 is likely due to a significant group of users delaying the update “after reading about how radical the changes were,” according to Millennial Media’s Gillis.

That awareness also drove a big change in after-work-hours iOS 7 downloads. “While some consumers may have downloaded the update during work, they most likely saw the major changes and decided they needed to explore the new system away from the pressures of the workday,” Gillis wrote.