2017 has been a bumper year for Steam. In April, Valve revealed that the platform was peaking at an average of 14 million concurrent users per day, which seemed to me like an incomprehensibly large number. But it looks puny in comparison to the numbers it's pulling in nowadays.
In September it hit 15 million, in October 16 million, and yesterday it smashed through the 17 million mark, reaching a high of 17,686,886 players logged on at once. Crikey. It peaked at 6.9 million players in game, rather than just using the Steam client.
Presumably Black Friday has something to do with that, and you won't be surprised to hear that the meteoric rise of PlayerUnknown's Battlegrounds was a big help. With nearly 3 million concurrent players yesterday, it made up a big chunk of the numbers. It won't be long until PUBG breaks through that 3 million mark for the first time. That's remarkable considering that in September everyone was making impressed noises when it broke the record for concurrent Steam users at 1.3 million.
For Steam, today wasn't quite so massive, but it still broke through the 17.5 million mark. It will likely die down after the weekend (weekdays are naturally quieter) but it wouldn't surprise me if it reaches 18 million within the next few weeks.