It goes without saying that there are a lot of difficulties inherent in attempts not just to measure Ubuntu's market share, but to define what "market share" itself means. For example, are you an Ubuntu user if you install it on an old PC and use it once in a while, or does it need to be your main operating system? What if you dual-boot? Since Ubuntu doesn't cost money, can it be said to have a share of the operating-system market at all, or does it fit in somewhere else?
What Canonical saysThe figure that seems to be most ubiquitous for quantifying Ubuntu's user base is 8 million. This number dates to a December 2006 interview in which Mark Shuttleworth, Ubuntu's founder, stated, "We know now that there are probably at least 8 million users." Unfortunately, he didn't explain how he arrived at that figure.
Curiously, Shuttleworth revised his estimate in October 2007, when he asserted that Ubuntu had "in excess of 6 million users," again without outlining the methodology behind the approximation. And while "in excess of 6 million" could be compatible with the claim of "probably at least 8 million" made ten months earlier, that's a pretty wide margin of error.
To complicate matters, Canonical representative Chris Kenyon again cited the 8 million figure in October 2008, admitting that market share "is a hard thing to count and there are lots of issues about methodology for counting but I have seen nothing that sheds doubts on that."
Given the variance in popularity estimates put forth by Canonical and the absence of a transparent process for measuring market share, the only certain thing to take away from these numbers is that no one at Canonical really knows with meaningful precision how many people are using Ubuntu--or if they do, they don't want to make an exact number public.
DistroWatchBeyond Shuttleworth's nebulous figures, a second source for estimating Ubuntu's popularity is the venerable DistroWatch.com. The site's "Page Hit Ranking" lists Ubuntu as about 1.5 times as popular as Fedora, at least among people who visit DistroWatch. Of course, no one knows exactly how many Fedora installations exist, but the project offers published statistics that can be useful for estimating.
Extrapolating Ubuntu's market share based on DistroWatch's numbers and Fedora's statistics is an exercise left to the reader, however, because I couldn't figure out any way to do it reliably or meaningfully.
Dell NetbooksDell's sale of netbooks, one-third of which reportedly ship with Ubuntu, also sheds some light on how many people use Ubuntu. Unfortunately, Dell doesn't say how many netbooks it sells, and critics have argued that many people may buy the Linux version of the Dell Mini to save money, and then replace Ubuntu with Windows when the machine arrives. (I suspect that such customers are a minority, since non-geeks don't reinstall operating systems; they use whatever comes with their computer.)
Given Dell's dominance as a PC vendor in most markets, and the fact that netbooks have been a hot item over the last year, Ubuntu's popularity on Dell Minis would seem to point to a large user base. But it's difficult to say anything more precise without more information from Dell.
ConclusionIt's clear from the approaches above that putting a hard number on Ubuntu's market share is impossible. Canonical seems reluctant to put forward any qualified figure, which is unfortunate--minimally, it would be nice to see an honest attempt at transparent analysis similar to Fedora's statistics.
In any case, perhaps it doesn't really matter how many people use Ubuntu, as long as those who do use it like it. Popularity isn't a measure of quality. But it is a good indicator of progress towards solving bug # 1.