The best example would be the creator of 'Flappy Bird'. He did earn a pretty neat amount of money from a unique free game that made the people go nuts about it. His game went viral but he then removed his game due to that earning a lot will make him go crazy and different from himself. He didn't become a millionaire but he was sure earning a decent amount of money. It is in the concept of the app that would make your product be known by many.