It receives its official release right on the doorstep of The International this August, which hosts the largest tournament in the DOTA 2 calendar. It's Steam's top played title.
It enjoys a multi-million dollar economy all to itself and that's before Valve have even aggressively marketed it. DOTA 2 will release as a free-to-play title, so expect a huge surge in players.
Previously Valve had stated that it would only release fully once it had matched the original DOTA's hero roster, but it's still a little off. Obviously they're comfortable enough with it to see it launch ahead of time.