Apple's OS X Mavericks update, which was released for free on Tuesday, is already installed on 5.5 percent of all Macs and rising quickly.
That's according to advertising network Chitika, which says the OS is seeing three times the adoption of last year's OS X 10.8 "Mountain Lion" update during the same first 24-hour period.
One simple reason for that? The software update, which Apple says brings more than 200 new features to Macs, was given out for free. That's a first for Apple, which charged existing Mac owners $20 for last year's update, and even more for some previous versions.
Chitika's numbers, which are based on millions of ad impressions on Web sites in North America, are less than half that of the 11.6 percent adoption rate reported by GoSquared, which has a live tracker. Apple could provide its own, official download figures as soon as this week, as it's done with previous releases.
At last tally, Apple's Mac user base was 72 million users, though that number could be larger. Apple reported that figure at WWDC in June, and it was double the figure from five years before. Between then and now the company's announced sales of an extra 3.8 million Macs, and will report additional figures for its September quarter next week.