These people keep bringing up "mArKeT sHaRE"
EDIT: I wonder if Microsoft are making anti-Linux accounts to try and hamper Linux.
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0
Not all Linux distributions are free. A market share can also be taken by a product which is free. It's just a piece of the market.
You don't count OS market share by the amount of money you spent for an OS (because then, MacOS would be free, since it's technically given away bundled with Apple hardware) but by the number of installs.