Steam Linux usage -- when measuring up all of the Linux distributions reported -- was right around 1.1%. Of course, there's some margin for error with the "Other" OS not being broken down, this being a random sampling of Steam gamers participating in the survey, etc. In November the result was right around 1.0%, so basically there was no major change or only a small rise over the past month.
SteamOS arrived in mid-December and it's only at a 1.0 Beta stage so it didn't really impact the results much for December. Given the early state of SteamOS and it not adding too much value yet over what can be found by installing Ubuntu / Debian / Fedora / openSUSE/Gentoo and playing your favorite Steam games, I wouldn't expect much immediate rise in the Linux numbers. There's also only a small number (circa 300) Steam Machines running out in the wild.
The Steam Linux usage should almost definitely rise later this year when more Steam Machines begin to surface -- from both Valve and various IHVs. Those Steam Machines should be running the Debian-derived SteamOS. We may also see the Linux desktop slowly improving too, which could equate to a rise in Steam Linux use as more gamers are ported over and gamers have less dependence upon Microsoft Windows or Apple OS X.
Those wishing to see the December 2013 Steam Software/Hardware Survey numbers can visit the SteamPowered.com survey.
If you missed our earlier exclusive coverage on SteamOS at launch, check out the SteamOS vs. Ubuntu benchmarks, SteamOS vs. Windows 8.1 benchmarks, Intel HD Graphics SteamOS testing, and a NVIDIA GeForce comparison on SteamOS. There were also many other SteamOS Phoronix articles in the past few weeks.
What's your hopes for Steam on Linux, SteamOS, and more broadly Linux gaming for 2014? Let us know in the forums!