Dota 2 playerbase actually declined in September, massively
It's by no means time to ring the death bells for Dota 2, but the game has recently witnessed a very sharp decline in its playerbase.
The most recent update to Dota 2 brought the whole game into the Source 2 engine. The Reborn update, which came out officially in September, was anticipated by players due to the improvements it promised, but was actually followed by a drop in the number of people playing the game.
The data on Steam Charts for Dota 2 reveal the game lost a little over 16% of its players. That's 510,522 in September from 606,944 in August.
Going as far back as July 2012, the game has never went above a 9% loss rate.
The reasons for this decline can be hard to pin down, but looking over the game's sub-Reddit, it's easy to see a staggering number of bug reports. The game's steam forums also offer a similar look.
One other contributing factor that would certainly have an effect, albeit not as pronounced as bugs would, is that The International - Dota 2's biggest eSports event, finished in September.
Thanks, NeoGAF.