Dota 2 officially holds the record for overall concurrent players for a steam game — an odd accomplishment given that the game isn't even officially out yet. I suppose it's a testament to the Dota brand that it can amount this type of presence without an official release.
Over the weekend it was spotted that the overall concurrent players for Dota 2 peaked at 297,116 concurrent users, slightly higher than the former record-holder Skyim (287,411). To be fair, Dota 2 — despite being in the "early access" stage — is free-to-play and players can easily get hold of a free beta code.
297,000 concurrent players is quite a feat, not just for a MOBA. In fact, we should actually be less surprised that Dota 2 was able to accomplish this. Given the breakout success of League of Legends, another multiplayer online battle arena game, and the popularity of Dota 2, it's easy to see that the MOBA genre won't be slowing down anytime soon.