Up until now, it was possible to save your Merge Dragons! progress only via Facebook to make sure it was saved. As you know, saving your progress gives you a possibility to change between devices. 
As of version 4.11.0, you will be able to save your game progress via Facebook AND Apple Sign-in!

If you have an IOS device and if it supports OS 13 and/or higher OS, you will have both options to save your progress through the Cloud Settings.
If you've already linked your game instance with an account, you won’t be allowed to log in with another account ( if you have multiple Facebook accounts).
You cannot use multiple Apple accounts linked to the same game.
You cannot use multiple Facebook accounts linked to the same game.
1 device = 1 Facebook & Apple ID

When you save your game via Apple Sign-in, you will be able to switch your accounts only on your IOS devices (iPhone and iPad).

If you have both Android and IOS devices and want to save your account between these, you need to connect via Facebook. Please, don’t delete your Facebook account if you thought about it.