Steam's Linux survey results for April 2014 reflect around 1.26% market-share for Linux, which is a slight rise over previous months in the ~1.1% range. Officially, Valve reports a +0.06% increase in the Linux market-share on Steam. Of the reported Linux distributions in the survey, Ubuntu Linux ultimately remains most popular -- in particular Ubuntu 13.10 64-bit.
Of course, it's worth reiterating the usual caveats that the Steam Hardware/Software Survey is only a random sampling of Steam gamers, some believe the system is somehow "rigged", etc. Most also aren't expecting any huge increases in market-share until the Steam Machines start appearing in large numbers, SteamOS becomes stable with some compelling reasons to install, and/or there's some Linux time exclusive launch title or other compelling reason to hop from Windows to Linux.
The Steam Survey data can be viewed at SteamPowered.com.