Valve is a company that knows how to sell games. Just look at Portal 2's impressive sales numbers, or how badly everyone wants to find a copy of The Orange Box, even years after its release.
Or, for that matter, the Left 4 Dead series, which has proven most profitable. Valve writer Chet Faliszek talked with VG24/7 recently, explaining how the series has sold over 12 million units to date, even without a whole lot of marketing.
Chet explained that Valve didn't do much with marketing because "when a game does really well at pre-release, they're going to know that you're not just talking a bunch of PR crap."
He continued, "We just let the game speak for itself because the Internet has made this thing where you can't pull the wool over people's eyes any more. They're going to know you're hyping." Hmmm, a slight jab at Capcom and Resident Evil 6, perhaps? Nah.
Anyway, congrats to Valve for sales well done. Maybe now they can work on something with a certain 3 in the title. Hey, we could be talking about Left 4 Dead 3, y'know…