If you created your account using an email address, you'll be able to link it to a Facebook profile in Menu>See profile>Edit>Facebook and tapping on "Link".
If you have a linked Facebook profile and you wish to unlink it, follow the same steps as before and tap on "Unlink".
Bear in mind that if you wish to link your Facebook profile and you've not logged in yet, you'll need to log in with your email address and password and, once you've accessed your account, follow the aforementioned steps. If you click on "Log in with Facebook" directly, the game will create a new account from scratch associated with the profile you log in from.