Apple's Mac share of personal computers worldwide fell to a five-year low in December, mimicking the company's own numbers that have portrayed a four-quarter sales slowdown.

According to web analytics vendor Net Applications, Apple's desktop and notebook operating system -- formerly OS X, now macOS -- powered just 6.1% of all personal computers last month, down from 7% a year ago and a peak of 9.6% as recently as April 2016.

Net Applications measures operating system user share -- an estimate of the proportion of all personal computer owners who run a device powered by a specific OS -- by counting systems whose browsers reach websites of its clients.

The Mac's 6.1% user share in December was the lowest mark recorded by Net Applications since August 2011, more than five years ago. Although the Mac number fluctuates month to month -- as do all estimates from the metrics firm -- its general trend has been up for two-thirds of a decade: In 2009, for example, the Mac accounted for only about 5% of all personal computers.

User share does not equal sales -- the former resembles the installed base more than anything -- but Apple's own data fits the view of a shrinking Mac. In October, the company reported sales of 4.9 million Macs for the September quarter, a 14% year-over-year decline and the fourth straight quarterly downturn.

Apple's sales slide during the past 12 months has been steeper than for the personal computer industry as a whole, according to industry researchers from IDC and Gartner, a 180-degree shift from the prior 30 or so quarters, when the Mac's growth rate repeatedly beat the business average. The long string of successes selling Macs -- fueled by Microsoft's stumbles with Windows 8 and a race-to-the-bottom mentality among rival OEMs (original equipment manufacturers) -- resulted in Apple's computers gaining user share.

But the converse has been true more recently, meaning that as sales of Macs fell faster than of Windows PCs, Cupertino's share shrunk as well.

As that slippage took hold, two rivals -- Goliath Windows and David Linux -- both benefited with increases of around a half percentage point during 2016. Linux, the open-source operating system whose users are even more passionate when defending their choice, climbed out of the under-2% cellar last year. Net Applications pegged Linux's user share at 2.2% in December, slightly off the 2.3% peak of November.

Linux first cracked the 2% barrier in June.