You can only use Facebook to sign in if you either used it to sign up or you later linked your DirectAffinity account to your Facebook account.
If you are not in one of the previous cases, you must first signed in using your e-mail and password. Then, go to Settings > Login Facebook to link both account between themselves. You will then be able to use this sign in method.
You can unlink your DirectAffinity account from your Facebook account at any time using the options in Settings > Login Facebook.