From - http://xbox.joystiq.com/By: Alexander Sliwinski
Left 4 Dead on PC appears to receive a shot of zombie reanimation every time Valve's Steam service does a promotion, according to proprietary Xbox Live and PC GamerDNA statistics. The chart above, which is calculated based on "equal percentages of Xbox and PC users," shows that engagement on both platforms was almost equal for three months after release. The PC receives its first major bump during Steam's 50 percent off sale in February. Both platforms received a boost with the release of the free Survival Pack, but then engagement spiked dramatically during Steam's "Freaky Free Friday" promotion and stayed strong.
Sam Houston, community manager at GamerDNA, tells us that typically the engagement graphs for games "start high and just drop off." He notes that Steam sales and promotions can obviously give a shot of adrenaline to a game's engagement. He questions, "Could Microsoft do something like that? Then it would appeal to both platforms pretty highly. But, you know, Valve owns Steam and the game, so they have more flexibility."