But there are signs that OS X Snow Leopard, an edition shipped in August 2009, may be the Mac's equivalent of Microsoft's Windows XP, an OS that stubbornly refuses to go away.
Mountain Lion, also known as OS X 10.8, accounted for 25.8% of all Mac operating systems during October, according to statistics from metrics company Net Applications. That represented a three-and-a-half-point increase over September.
Apple issued Mountain Lion on July 25.
For all its gains since then, however, Mountain Lion has not kept pace with the uptake trajectory of Apple's last two editions, OS X 10.6, aka Snow Leopard, and OS X 10.7, better known as Lion.
Those two grabbed slightly larger shares after three full months of their availability: Snow Leopard accounted for 27% of all Macs by the end of November 2009, and Lion, which launched in July 2011, had a 26.4% share in October 2011.
Mountain Lion's gains were again more at the expense of Lion than Snow Leopard, although the gap narrowed in October.
While Apple customers running Snow Leopard can upgrade to Mountain Lion - assuming their Macs meet the requirements - they have done so in far fewer numbers than those who relied on Lion.
Since Mountain Lion's debut, Snow Leopard has lost 6.6 percentage points, a drop of 17%. Meanwhile, Lion has lost more than double that - 15.6 percentage points - falling 33% since July.
Snow Leopard has lost more than half its share of all Macs since Lion's appearance over a year ago, but so far it has been resistant to Mountain Lion's call to upgrade. In each of the last two months, for example, Snow Leopard's losses were less than its 12-month average.
Apple also, perhaps just temporarily, extended security support for Snow Leopard when it issued a patch update for the three-year-old operating system in late September, confounding security professionals who had assumed it would stop serving OS X 10.6 with updates, as it had done with earlier editions once two newer versions had been released.
Snow Leopard is no Windows XP - for one thing it's less than one-third as old as that 11-year-old OS from Microsoft - but the comparisons, what with both posting slow-but-steady declines and their makers' extending security support, are intriguing.
It's unclear why Mac users are holding on to Snow Leopard, but one factor may be that it is the newest Apple OS able to run applications written for the PowerPC processor, the Apple/IBM/Motorola-designed CPU used by Macs before Apple announced a switch to Intel in 2005. The first Intel Macs launched in January 2006.
(Snow Leopard will not run on PowerPC-equipped Macs - the last edition to do so was 2007's OS X Leopard - but it can run applications written for that chip via the Rosetta utility.)
Another possibility: Many Mac users dinged Lion for being less stable and reliable than Snow Leopard, and said they would stick with the older OS. Those sentiments have also been popular with many Windows XP users.
"I think [Snow Leopard] was the best OS they've made so far and I have no desire to turn my laptop into a smartphone by installing Mountain Lion," said one Computerworld reader in a comment appended to an Oct. 2 story, referring to several features that the latter shared with iOS, Apple's mobile operating system for the iPhone. "If I wanted a smartphone I'd already have one."
Net Applications measures operating system usage by tracking unique visitors to approximately 40,000 Web sites. More information about its October stats - including gains by Microsoft's Windows 8 - can be found on the company's website.
