It's no wonder that PlayerUnknown's Battlegrounds has overtaken Counter-Strike: Global Offensive on Steam's most played games list, since most of the current PUBG players are coming to the game straight from CS:GO. H1Z1 players are also slowly moving over to PUBG.
We had a suspicion that PlayerUnknonw's Battlegrounds players must be coming from somewhere, and a few months ago it didn't seem like they were syphoning players away from H1Z1. This is slowly changing now. AltChar's Ranum, usually a dedicated CS:GO player has also switched to playing a lot more PUBG than his previous favourite. It would seem that he isn't the only one doing so.
More than a half of all PlayerUnknown's Battlegrounds players are currently coming out of the Couter-Strike: Global Offensive player pool. A decent number of H1Z1 players seems to have caught on to the now unmissable PUBG trend as well.
The battle royale shooter currently holds the top spot on Steam's most played games stats. No conclusive data has been published yet regarding how many of PUBG's current player base could have come out of DOTA 2, but since the two games aren't quite in the same genre, it might not be likely.
PUBG is currently somewhere half way between 8 million and 9 million units sold, and gets well over 800.000 peak daily concurrent users. With more maps, content and updates on the way, and considering that the game is still in Early Access, it would take a minor miracle to get a Valve game back on top of the charts. Too bad Valve can't count past two.