El Capitan, also identified as OS X 10.11, was released on the last day of September: 61 days later it had been installed on 38% of all Macs, according to analytics firm Net Applications. That was an increase of 11 percentage points from the month before, when the upgrade set an adoption record for a Mac operating system.
Net Applications estimates operating system shares by tallying unique visitors to its clients' websites. In the absence of definitive data from Apple, that user share is one of the few proxies for real-world OS X adoption.
It's open for debate whether El Capitan's first two months generated a faster uptake tempo than its immediate forerunner, OS X Yosemite. The latter shipped Oct. 16, 2014, and at the end of November of that year, Yosemite had accrued a 37% user share. But because that timetable represented just six and a half weeks, not the eight and a half of El Capitan, the comparison was not apples-to-apples.
If the comparison instead used the end of the second full month of availability, Yosemite (with a user share of 44.5% at the end of December 2014), trumped El Capitan (user share of 38% at the end of November 2015).
As in the first month of El Capitan, November's growth seemed to come at the expense of Yosemite, whose user share fell by 7 percentage points. Other, older versions of OS X also lost user share last month, but at rates about equal to their 6- and 12-month averages, signaling that those declines were organic and driven by people replacing aged Macs with new machines.
A sizable number of Macs continued to run outdated editions of OS X last month, even though many of them were eligible for the El Capitan upgrade. By Net Applications' data, about 14%, representing one in seven Macs, was powered by a version that Apple no longer supports with security updates. Apple distributed the final security update for the three-year-old Mountain Lion in August, but continues to patch Mavericks and Yosemite.
The free El Capitan upgrade can be obtained from Apple's Mac App Store, and supports iMacs as old as mid-2007, MacBook Pro notebooks from late 2007 on, and MacBook Air laptops from late 2008 going forward.