First game on Steam to break a million. http://www.bit-tech.net/news/gaming/2015/02/16/dota-2-hits-1-million-concurrent-players/1
Er... 'no particular high profile event'? Actually the 'Beast Brawl' event went live over the weekend, and is only available during random 10-min windows, which is almost certainly why & when those million people were online at the same time (causing some controversy and also crashing DOTA's servers a fair bit too).
^Yep Stayed off Dota this weekend apart from 1 or 2 games during big offpeak times because the servers simply weren't coping. Either you couldn't connect, or you spent so much time waiting for a game you gave up (had one where I lost track of the time while browsing Reddit, only to check back and it was still counitng at 35mins)
Very true on Year Beast Brawl, that pushed the number over 1 million for sure. Had some troubles with my first que Friday but ended playing two brawl events over the weekend.
The fact league has 5-7 times this number on stable servers suggests valve have along way to go if they want to dethrown league of legends.
Defense Of The Ancients Multiplayer Online Battle Arena Not mentioned, millions of mice with broken left clicks.
Think LoL was right click, regardless most people throw their mouse/keyboard/monitor/pc out the window before any of them wear out while playing it! Smite was the most interesting IMO.