It seems to have plateaued and increasing more slowly. Combining data from Steam and Statcounter reveals this:
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
Yes, GNU/Linux has a slow steady growth. Windows had >90% market share in 2009, now it has around 72%. Of course, some went to Mac, but many people to gnu/linux, too.