Valve is pretty popular. So too are MOBA games like League of Legends. So I guess that explains why there's now over 150,000 people playing Valve's DOTA 2 at any given time before the game's even technically out.
As those without access are aware of, you can only get the game if you're invited, as Valve continues one of the longest and most public betas since Gmail. Sure, those invites aren't exactly hard to come by lately, but still, there's a big difference between something that's restricted and something that anyone can download at any time for free.
Under those conditions, the concurrent player base for DOTA 2 has now hit 180,000. That means there are 180,000 playing it at once.
Imagine how many will be playing when the game's actually out.