The Left 4 Dead series has sold more than 12 million copies, Valve writer Chet Faliszek noted in an interview with VG24/7. Faliszek talked about the company's approach to promoting its games, saying Valve doesn't rely much on 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."

"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," he said. Either the zombie apocalypse creates some high-quality wool, or Left 4 Dead must be a pretty great series.
http://www.joystiq.com/2012/09/30/le...an-12-million/

This and Dead Rising are my two favorite zombie games. Nice sales on PC/Xbox. Wow.