I think checking the Steam concurrent gamer numbers is becoming part of my daily ritual, with PlayerUnknown's Battlegrounds now pushing a huge 2.43 million concurrent users on Steam.
At the time of writing there were 791,589 current players and a new peak of 2,437,035 players. Comparing this against the mammoth Dota 2, PUBG blows it away, as Dota 2 only has 325,206 current players and 677,750 peak players. Another FPS powerhouse is CS:GO, which only has 159,526 current players and 486,491 peak players.
The first, second, and third positions have hundreds of thousands playing at once and for PUBG's case, millions playing at once. The games below that like TF2, Warframe, PAYDAY 2 and GTA V all have considerably less. PUBG truly is going crazy, where I think we'll see 2.5 million concurrent players over the weekend and then its into the 3 million stretch from there.