You should be working on trying to take back ownership of your relationship with your users from Facebook, and remove the API from any future business plans. I

You should be working on trying to take back ownership of your relationship with your users from Facebook, and remove the API from any future business plans. Implement your services off of Facebook, and email your app users telling them your services have migrated to your website or mobile app. If it wasn’t obvious already, the API is being ground into dust, and it will only further deteriorate after GDPR takes effect.