A few weeks ago at COSSFest in Calgary, I heard a talk on this subject by KDE's Aaron Seigo. Here, I recreate his logic from memory and imaginative extrapolation, so my apologies if I get any parts of it wrong. The point is the general argument, anyway.
Seigo began his estimate by a story about how a scientist in the 1940s supposedly estimated the force of an atomic bomb by measuring how far it displaced scattered scraps of paper. He tells this story to emphasize that he derives his estimate from indirect indications, rather than hard figures or direct observation.
Observing that North American trends are not always in keeping with those of the rest of the world, Seigo points out the countless indications of growing GNU/Linux adoption: data points like the growing number of computer manufacturers offering pre-loads, and the operating system's success on netbooks.
He talks about cities and government departments in Europe adopting GNU/Linux, of 53,000 labs in Brazil deploying the operating system to 52 million students, of the dominance of Red Flag Linux in China, and similar success stories throughout India and South America, pointing out the rapid growth among populations equal in size to North America and often larger.
Look outside North America, he says, and the generally accepted figures are almost all too low, distorted by the Windows-dominated perspective of the United States and Canada.
At the end of his presentation, Seigo was reluctant to give a percentage, but, when I pressed him, he suggested that 8% was probably a bottom figure, and 10-12% the likeliest. Some of the audience seemed politely skeptical, but, even if you half his figures, his examples go a long way towards suggesting that North Americans consistently under-estimate GNU/Linux use because we live in a Windows-distorted environment.
Some of his scenario seemed too rosy. For example, he did not discuss how, so far as the Chinese government is concerned, one use of Red Hat Linux seems to be to force Microsoft to lower its license fees. Nor did Seigo address claims that many GNU/Linux boxes distributed in Brazil soon have copies of Windows installed on them.
Still, even if you doubt Seigo's exact figures, his presentation did suggest that, whatever the user statistics are now, they're about to rise sharply.
The wide variation in the figures that I've mentioned only emphasize the difficulty of determining user numbers. Apply a different set of assumptions, be stricter or laxer in your estimates, and the numbers can vary wildly.
All the same, exploring the possibilities is still worthwhile, if not for the original purpose of establishing hard figures. Instead of giving an exact number, running through the logic, the assumption and the biases behind the numbers is a useful reminder that the objectivity of numbers is often just a myth.
Are GNU/Linux desktops as rare as NetApplications suggests? As widely deployed as Seigo would have them? We have no way of knowing which is closest to the truth.
But at least Dotzler and Seigo, free software advocates that they are, explain their reasoning, instead of just offering a number with the illusion of objectivity that media services and columnists will pick up without asking questions.