In November, the Steam Linux use was at 0.98%, while for December it pulled back to 0.97%.
Gaining a bit was Ubuntu 15.10 while the other distribution use was unchanged or shrunk according to Valve's metrics for the past month.
You can see all of the December 2015 Steam Survey results in full via this page. Linux was at 0.97% while Windows was at 95.33% and OS X at 3.62%. No major change to end out 2015, but not really too surprising considering there hasn't been any super exciting Linux game exclusives, the lack of AAA games compared to Windows, the various driver difficulties, and lack of other compelling reasons for Windows gamers to make the switch to Linux at this time. Hopefully this number will creep higher in 2016.
On a related note, as of writing this article there are 1,756 native Linux games available on Steam.