Zynga is considered the winner not because it milks the most profit per customer but because they have the most popular games. So really it's not so much about

Zynga is considered the winner not because it milks the most profit per customer but because they have the most popular games. So really it's not so much about getting people to pay - it's about getting people to play. FarmVille is famous because 84 million people play it. World of Warcraft still makes more money than FarmVille, but World of Warcraft has nowhere near that userbase.