When you log in with Facebook, the game actually creates a brand new account for you.
Your previous progress still exists but as a separate account. You can still access this account if you log out of Facebook.
If you had a lot of progress on your account before using the Facebook login and wish to transfer that account to the new one, please write an email to the customer care from within the game.
To do so, use the "Email us" button situated in the Parameters menu.
It is essential that you do not change the text that is already filled out in the email for you.
This number is your Account ID and it is the only way customer care can help you with your issue.
Customer care will require the account IDs of both your clients to do the transfer.
To get both of them, you need to log out (or login) and then write down the number automatically entered in the email form for you.
After you send your request, the customer care should get back to you shortly after.