Facebook is committed to providing the best user experience possible and we continue to investigate ads that violate our policies. We also encourage developers and ad networks to maintain high quality apps and advertising practices and may take further action against developers who host ads that violate our policy.
If you have confirmed that the advertisement is running within an external app, you can report this to the developer by going to the app's profile page and clickingContact Developer towards the bottom of the left column, or by clickingReport/Contact this App at the bottom of any canvas page within the app. You will then see an option to contact the developer directly.
If the advertisement is on Facebook, please follow the instructions listed here to report the advertisement.