Steam just hit new highs with its user base in November 2017! There were around 7 million players playing games at once on their platform which is really hard to imagine.
The increase in concurrent user base that just hit amazing number of 17 million was probably caused by all the “black friday” deals. Also worth mentioning is that almost 3 million players out of 7 million were playing PUBG (Player Unknown Battlegrounds).
More details on: http://store.steampowered.com/stats/