Well yeah, but... Most of those users use it for software, not games, right ?
Actually, Games is the most popular category. Though yes, users also use the store to get software. But software is also being sold through Steam now, are you going to exclude Steam, if you're excluding Windows Store, because it also offers non-games?
Since we're here, what about the Apple guys? The iTunes store also sells games, and the iPad is nothing more than a personal computer in a tablet form. Are we going to count iTunes here?
EDIT: Battle.net sells first-party games only, yet it makes more money and has more users than GOG. Are we going to exclude that one because it doesn't sell third-party games?
Lots of variables that need to be considered, thus, you can't just simply put a list of most popular / most used / most profitable / whatever DD services.