There is a temporary Facebook bug that is not allowing mobile users to access Facebook apps. The temporary workaround for this issue is to publish your promotion to microsite and mobile. When linking to your promotion app – provide users with the microsite/mobile url instead of the Facebook url. This microsite url will automatically redirect to a mobile optimized version for users accessing it via their mobile device. Please note that like-gating will not be enabled for the microsite/mobile link.
Votigo is proactively communicating with Facebook and exploring all opportunities within our technology to resolve the issue as quickly as possible. Details of the bug can be found here: https://developers.facebook.com/bugs/255441881241347