Previously, Skyrim held the record for the game with the most concurrent players. Set during its release month in November 2011, Skyrim peaked at 287,411 players. Though, this number rapidly slopes off, this month, for instance, only saw 44,037 concurrent adventurers in Tamriel. Dota 2, conversely has ramped up its playerbase steadily, again, since November 2011.
This month it finally overtook Skyrim's record, setting a new peak at 297,010.
You can see the graph better here.
Also, because Dota is still in beta these numbers suggest its playerbase is only going to continue rising. Valve must be pretty excited to see what player numbers they'll be seeing when they actually get round to opening it up to everyone.