Over the last year, Steam on Linux usage appears to have been slowing. Stats from March show that this trend is continuing, as usage of the Linux client declined by 0.06% down to just 0.85%.
Valve's own project SteamOS runs Linux; however, devices using the operating system have not been prompted to take part in the survey process yet, therefore the Linux usage is not properly being represented. Additionally, Steam's figures show Linux usage share relative to the total amount of users, rather than reporting actual figures, which might show a growth in Steam for Linux adoption which could have been overshadowed by a general influx of new users on all platforms.
Unlike Linux, the 64-bit version of Windows 10 is storming ahead and stealing the market share of older versions of Windows. March saw Windows 10 increase its usage by 2.97%, putting it at 38.28% with Windows 7 leading slightly at 39.96%. Overall, Windows saw a +0.11% change, leaving it with 95.70% of the overall market share. OS X based systems remained relatively stable, dipping 0.08%, putting it on 3.32%.
Update: Adjusted figures to take into consideration 32-bit versions of WIndows 7 and WIndows 10.