One factor that will limit Mountain Lion's eventual top mark was Apple's announcement last February that OS X would adopt an annual release cycle, meaning that, like Lion last summer, Mountain Lion will soon be superseded by something newer, probably this summer.
Microsoft has reportedly also assumed an annual release schedule for Windows, with the first instance of that express cycle slated to ship around the middle of this year.
Net Applications measures operating system usage by tracking unique visitors to approximately 40,000 Web sites. More information about its December statistics can be found on the company's website.
OS X Mountain Lion's share of all Macs climbed to 32% last month, making it the most-used version of Apple's operating system five months after its release. (Data: Net Applications.)
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is email@example.com.