If you receive a message indicating that the current account is bound when attempting to bind your base, first log out of all your Facebook accounts, whether from the browsers on your device or the Facebook app itself and stay logged out.

Then, clear the history for the Safari browser by following these steps: 

-Open the Safari browser on your device.
-Tap on the book icon at the bottom center of the Safari screen.
-Tap on the word 'Clear' at the bottom right of the screen.
-Afterward, exit the game and open it again, then try binding again using Facebook and manually enter your information.