The concurrent users record was broken on Steam earlier today.
The game being played most at the time was Dota 2 with 951,942 users, followed by Counter-Strike: Global Offensive with 675,195 and GTA 5 with 116,230 users.
Earlier this year on January 2, there were 13,910,000 concurrent users on the service which was an all-time high.
As of press time, there are 12,978,881 folks currently using Steam the game with the most folks in it is CS:GO.
I'd make some sort of nerdy joke about it, but since I'm one of those using it at the moment, I won't.