A 0.05% drop is well within the expected margin of error with a perfect statistical methodology and large sample size. The steam survey is opt in, it's a good rough guide but you can't read anything into month-month changes.
Even if we generously agree that it's correct, how much has steam grown? A 0.05% drop could still be more total users than the previous month.
The usage share was 0.97% at the end of 2015, 0.87% at the end of 2016 and 0.75% in the latest survey. This might still translate to a growing absolute number of Linux users, but it would be nice to see the momentum start going in the opposite direction when it comes to those percentages.
Even if we generously agree that it's correct, how much has steam grown? A 0.05% drop could still be more total users than the previous month.