No businesses release their sale data, really. Only as much as is required by law, for like shareholders and such. Sales info is considered propriety data, (they don't want competitors to get ahold of that info) if any employee who knew it released it publicly they would be fired and likely sued by Marvel. The numbers that get argued about on forums are not 'released' by marvel, estimates made by comichron, trying to guess the sales.
Long story short, marvel absolutely tracks digital sales, but they ain't never gonna release those numbers. They'll only talk about them in generalities.