It's still got a while to go before it catches up to genre rival League of Legends, though;game has reached over 7.5 million concurrent users. Which is pretty ridiculous.
To give you an idea of just how ridiculous, Steam managed to hit overback when the Summer Sale was on last year. Not that Valve will be too disappointed in Dota 2's performance, even if it doesn't yet quite match up to LoL – those are still mightily impressive numbers.
They were likely given a nice bump by the ongoingevent, which offers a new character, the frost-happy Winter Wyvern, unique prizes, and the ability to summon and control a monstrous creature in mode.