Top Girl is a female-focused mobile role-playing game that allows players to create a fashionable avatar and then climb up the fashion social ladder, collecting money by doing modeling jobs, buying new outfits, and going to clubs. The app is free to play but charges users for virtual goods.
CrowdStar says that Top Girl has also became one of the top-grossing games in the App Store and is currently one of the top three grossing games, over Angry Birds. As I wrote above, Top Girl makes money exclusively from virtual goods.
To keep the download stat in perspective, Angry Birds Rio hit 10 million mobile downloads in 10 days.