It is possible to sign in using Apple ID if you wish. An existing guest game can be linked to an Apple ID but you can not link a Facebook to the same account. If your account is already linked to a Facebook you will not be able to link an Apple ID. If you have more than one device using your Apple ID, you will be able to play across those devices. We recommend reading the FAQ on playing across devices.
Please note that you should note down your unique numerical Runaway User ID for your game still because we will not be able to locate data using Apple ID.
To stop using an Apple ID with a game:
You will need to go to your device settings, then select your Apple ID profile. Next you should select "passwords and security" followed by "apps using your apple ID, select Furistas and choose to stop using Apple ID.