More consumers are cutting their consumption of carbonated soft drinks these days; sales volumes of the beverages declined last year 1.2%, an acceleration over 2014 when they fell less than 1%, according to the industry watchers at Beverage Digest. But diet soda sales in particular remain in free fall, and the two big soda giants, Coca-Cola and PepsiCo, continue to watch volumes of their respective zero-calorie brands slip away.