The Chicago Tribune reports that Diet Mountain Dew brings in $1 billion a year in sales. That's enough for PepsiCo, which produces the Diet Dew, to buy every many, woman, and child in America two 20-ounce bottles, with some change left over. And that 20-ounce bottle is very popular. Here's the Trib:
Diet Mountain Dew was introduced in 1988. According to the company, it is now the top-selling 20 ounce diet soft drink by volume in convenience stores and gas stations.
So you can see where the billion comes from. Interestingly, the other two billion-dollar Pepsi brands are Brisk tea and Starbucks beverages. Only one of those is an iridescent green color and contains few calories while invoking the storied history of American moonshine-making, so if that's what you look for in a drink, Pepsi has you covered.