When tallying up all of the different Linux distributions that made it into the September 2013 Steam Hardware Survey, the Linux usage resides at just about 1.0%, while not counting the "other" operating systems. This is basically the same -- if only maybe 0.08~0.1% higher -- than the month prior, which has dropped compared to initially when the Steam Linux client first rolled out and was around 2% Linux usage. You can fret around about some minor percentage differences if you wish, but that's where things are basically at today. These results can be found in full at SteamPowered.com.
At least going forward there's more and more native Linux games being offered on Steam. Valve isn't getting discouraged at all by the current Linux numbers as they continue bringing new and old titles to the "1%" open-source operating system. The Linux statistics will certainly be bumped up soon enough with Valve's Linux-based SteamOS and their SteamOS-based Steam Consoles/Machines.
I wrote last year that 2013 would be the year of Linux gaming and it certainly has been so far but even better announcements are yet to come this calendar year.