(Shown below are the relative market share of various OSes according to Wikipedia)
So Windows is the most popular OS. I think there's no controversy regarding that. What creates a controversy though is the market share of Linux. Now keeping a tab on Windows market share is relatively easy, since there is a definite sales figure (and perhaps tracking). But when it comes to Linux, because its free and no "buying issue" is involved, tracking the total number of users is extremely difficult. What complicates the problem further is the presence of literally hundreds of distros. Yes, Ubuntu is without doubt the most popular Linux distro (Distrowatch reports an overwhelming hits per day statistic in favour of Ubuntu), but its not the only one. Fedora is very popular among Linux users, and there is a dedicated fan following for OpenSUSE, Free BSD, CentOS.
(Shown below is a screenshot from Distrowatch showing average hits per day)
Tracking the growth of each of these distros requires an immense amount of coordination effort among companies and developers which unfortunately the Linux sphere has not seen in the past. Distro bashing these days is a favourite pastime for many and the importance of a collective effort to create a platform for collaboration and the bigger picture is missed altogether.
Canonical has recently brought a new tracking application to its distro Ubuntu which helps it to collect info about how long the OS is installed on the PC etc. As of now there is no detailed info about exactly what information Canonical is collecting and what it intends to do with it and as expected there this concern has been reciprocated by a few users. (See Slashdot and Phoronix more info on this)
So why is tracking all that important? Why is the marketshare such a hallowed statistic? Why does a normal user need to even bother about what percentage of users are using the same OS he's using? The answer has been eloquently given by Slashdot blogger Gerhard Mack who says:
"There are hundreds of conflicting estimates of how many Linux machines are out there. Knowing how many people are using Linux is important because larger numbers provide leverage when we go asking for drivers from hardware manufacturers, apps from software makers or try and convince OEMs to bundle Linux."
Linux needs more support from hardware manufacturers and drivers support to improve the "Linux experience", if I may say so. The only way to convince manufacturers and developers to do that is to give them hard data. Thats a fair argument I think, as long as the user is aware of the info that the company is collecting.
After all for most (if not all) Linux is much more than just an OS.