There are no brakes on the PUBG train. PlayerUnknown's Battlegrounds is now the most played non-Valve game on Steam, surpassing Fallout 4 which previously held the position, right behind Dota 2 and Counter-Strike: Global Offensive.
PlayerUnknown's Battlegrounds has recently sold 6 million copies, and it hasn't even properly launched yet. The immensely popular last-man-standing shooter has now marked the highest consecutive player number of any non-Valve Steam game on the platform. This spot was previously held by Fallout 4, at close to 500,000 concurrent players.
Brendan PlayerUnknown Greene has repeatedly said that the game won't spend more than 6 months in Early Access, but that promise has been shaken somewhat with recent delays. The rumour mill is spitting out chatter about a 29 August 2017 release, but more official statements by Bluehole Studio name only a vague Q4 2017 release date.
The only two games on top of the are two Valve giants, Dota 2 and Counter-Strike:Global Offensive. If PUBG keeps growing in popularity the way it is, it might not be too far of a stretch imagining it in one of the two first spots, especially because the two top games have been dropping in player numbers over the last six months, while PUBG was on the rise.
Bluehole Studio has recently announced that the game will be receiving a loot system for cosmetic items, and players weren't too happy about the news. The game has also been confirmed for an release during this year's E3.