Go on KissKissBankBank login page.
Then click on "Continue with Facebook". You will then be redirected to Facebook login page, or, if you are already signed in to Facebook, on the confirmation page to connect both accounts. That's it! There is nothing faster, and no more risk to loose your credentials!
Please note that all your data is protected, and KissKissBankBank will not post anything on your Facebook page without your consent.