The Steam survey data for last month shows Linux use at a 0.05% drop over the month prior, or a total overall market-share of 0.75%.
This is in comparison to ending 2016 closer to 0.9% and much earlier all-time highs of close to 2%. You can find some of the past data and commentary via our older articles. There are some that will argue the Steam survey is unfair to Linux users, among other conspiracies. There is also the fact this is on a percentage basis so the Linux gaming market may be increasing, but not as fast as Windows/macOS. The reported macOS marketshare for the month prior is 3.14%.
Those curious about Valve's Steam Survey data can find the updated figures via SteamPowered.com.