Like most industries, the gaming industry is bound by the conventional economic wisdom that you must spend money to make money. Historically, that's meant taking a loss on every game system sold (with the notable exception being most Nintendo consoles) in order to tap into selling game after game to console owners. This measure of success is known as the "attachment rate" or "tie ratio." A somewhat (November 2008) dated Gamasutra chart shows that the Xbox was in the lead, with 6.6 games/system sold, followed by the Wii at 5.5 and the PS3 at 5.3.
This statistic has historically been a powerful metric for measuring market penetration and overall success for a console. After all, what's the point of selling a console if you can't sell game after game? But as with many things in today's integrated media world, the lines have blurred and traditional metrics don't necessarily tell the whole story.