Steam Linux Usage Continued Falling In June
During April, the Steam Linux usage dropped below 1% and in May it dropped below 0.9%... In June, the Linux usage declined by another 0.01% to put the current Linux gaming marketshare estimate at 0.87%. Ubuntu and Mint numbers were lower while Arch Linux saw a 0.03% boost, according to Valve's numbers. This sub-0.9% marketshare is at a low-point especially compared to many months ago when Linux was around 2%.
These numbers probably won't elevate the Linux status any higher for the next few months until the premiere of Steam Machines, the stabilization of SteamOS, and the debut of more high profile AAA Linux games at or near launch time for Windows. This summer's debut of Windows 10 will also pose more competition and still the matter of (largely AMD) Linux driver quality. At least Valve continues to invest in Linux and OpenGL/Vulkan, etc, and appear committed for the long-haul.
You can look through the June 2015 numbers via the survey page.