The current split of Steam users, according to the Steam Hardware Survey, is 95.5% for Windows, 3.6% for Mac OSX, and 0.8% for Linux. Phoronix reports that this does not count SteamOS, and there might be other “inaccuracies” with the survey, but the Linux figures are 0.04% less than they were before (a relative drop of about 4.8%).
Windows users are up, and Mac OSX is flat.
A 4.8% drop in a month isn't promising, but it's also not too concerning. If you were intending to target a platform with 0.8% marketshare, then you can benefit from the long shelf life that Linux provides. It's not like a publisher is counting on that platform to reach two-week launch window sales figures. We'll see if the pendulum will swing back in the future, especially if Valve creates compelling, new, first-party content for Linux. They seem to be waiting to put their full weight behind it.