But it turns out everyone loves gambling, even if it's just for the experience. For Android, the highest average spend per buyer was for "Big Fish Casino," a virtual slots game. Big Fish is part of a subcategory of free-to-play games in which you're given a chunk of free credits to start with but have to start paying once you run out of the initial capital.

Remember that nobody can actually win a single dollar on "Big Fish Casino" because online gambling is illegal in the U.S. All the money spent by users is pure profit for the company. The users are buying vapor-worthy credits that serve no real-world purpose beyond furthering more Big Fish "gambling."

The in-app game space uses terms to categorize its users' spending habits borrowed from the language of casinos: whales, dolphins and minnows.

"We find that a game like 'Simpsons Tapped Out' has a fairly even distribution in those three segments in terms of their spending," Callahan said. "Whereas something like 'Game of War,' also a leading game, 80 plus, 90 plus percent of their revenue is coming from a whale—over $25 a month."

Social casinos (like "Big Fish Casino," "Doubledown Casino," and "Slotomania") are one of the fastest growing groups of gaming: revenue is expected to reach $3.5 billion in 2015, according to figures from Eilers Research, a firm that tracks the gaming industry. That's up from $2.8 billion in 2014 and $1.3 billion in 2012.



Churchill Downs, the company that runs the Kentucky Derby, acquired the maker of "Big Fish Casino" in December for $885 million.



Plus, the people who are into mobile casino games are really into them. The difference between mean and median for those games is greater than for other game types, meaning they have more skewed distributions. One iPhone user paid nearly $22,000 playing "Big Fish Casino" in the time period. (Just FYI, the central office for Gamblers Anonymous can be reached at (626) 960-3500.)

