http://www.pcgamer.com/number-of-active-steam-users-passes-18-million-30-more-than-a-year-ago/

More than 18 million people were logged into Steam at the same time yesterday, and more than seven million of them were in-game—both new records. Those are pretty abstract, albeit impressive, numbers, but if you compare it to this time last year then you'll see the number of concurrent Steam users has shot up by about 30%. That's a hell of a year for Valve. 

Most of the growth happened in the second half of 2017, as you can see by the graph below. In April Shaun was impressed about Steam pulling in 14 million peak users a day. That seems like a long time ago. It reached 15 million in September, 16 million in October, and 17 million in November. Yesterday it peaked at just above 18.5 million, so that growth is showing no signs of slowing down. At this rate, it should reach 20 million before April.