http://www.pcgamer.com/playerunknowns-battlegrounds-breaks-15-million-concurrent-users/

In early August, PlayerUnknown's Battlegrounds surpassed 500,000 concurrent players on Steam, which led me to wonder if long-time chart champ Dota 2 might be facing an actual threat to its position at the top of the heap. Not long after, the answer came back a resounding "yes," as PUBG very convincingly seized the crown, and then less than a week later stomped out Dota 2's all-time record of 1.29 million players.  

In case you were wondering, it's not slowing down. PUBG broke 1.4 million concurrents on September 22, and then hit a new all-time peak of 1,523,179 concurrent players one day later, according to Steam Charts. It's also still selling like gangbusters: SteamSpy estimates that it's moved another two million copies, give or take, since it broke the ten million sales mark on September 5.