Here's the latest Steam Hardware survey for December 2014, and Linux actually increased, but not by a lot.
Linux results for December 2014
Ubuntu 14.04.1 LTS 64 bit 0.42% -0.01%
Ubuntu 14.10 64 bit 0.14% +0.01%
Linux 3.10 64 bit 0.10% +0.01%
Linux Mint 17 Qiana 64 bit 0.09% -0.03%
Ubuntu 14.04.1 LTS 0.06% 0.00%
Linux Mint 17.1 Rebecca 64 bit 0.06% +0.06%
Total: 1.16% + 0.03%
Last Month: 1.13%
My thoughts on it
The biggest change as expected for December, was from some people (like myself) moving to the new release of Linux Mint. A small movement from Ubuntu 14.04 to it's latest release and to the mysterious "Linux 3.10" which is most probably Arch.... and that is it for this month.
Important things to remember
Be aware these results will probably not be that accurate as we don't know how they do their percentage results, they could be rounding up, rounding down or truncating the percentages. So a 0.5% could actually be nearly 0.6% as it could be 0.59% but they could do no rounding and simply truncate it.
Also remember it is a survey, so it won't ask every single one of you to do it. It would only be truly accurate if it did it behind the scenes, but that's not what a survey is for this is just to get a general idea.
Final Note: Look at it this way, Steam has around 100 million active users, 1% of 100 million is about 1,000,000 (1 million) people. What developer wouldn't want to hook into a market that big? Of course it doesn't mean they are guaranteed that amount of sales, but it's something fun to remember.
See the survey on Steam here.