You can only link one game per Facebook account. If you have an old city linked to your Facebook and want to add/replace it with a new city, you will need to make a new Facebook account.
January 2015 - last edited January 2015
So ounce you link a Facebook account it can never be unlinked?
Precisely. You can obviously bulldoze your city and build it again, but you'll still be the same level with the buildings/specialisations etc unlocked - you cannot go back to level 1 on the same account.