Valve has released Steam Hardware & Software Survey for December 2013.
According to the Survey 1.85% Steam users run Linux.
The most popular Linux distribution for running Steam client is the latest Ubuntu 13.10 64bit with 0.41% of share, while the next most popular Linux distribution is Ubuntu 12.04.3 LTS 64 bit with 0.19% share.
Valve is releasing monthly snapshot of hardware and software it receives from users who agree to share their data anonymously.
It will be interesting to follow Linux share as more and more games get ported / released for Linux.
Perhaps Linux won’t pick up as a gaming platform until Steam machines get publicly available, but one thing is for sure. Linux use will grow in 2014.