(1) Your app uses only Facebook Login to authorize people using your app.
(2) Your app provides two login options: Facebook Login and your own login mechanism.
(3) Your app uses your own login initially and Facebook Login is an option to switch on certain social features.
Facebook SDK provides a UserSettingsFragment class that can be hosted by your login activity to display the Facebook login/logout button. In case (2), the Facebook SDK provides a LoginButton class that provides a button you can drop into your UI. The LoginButton class maintains the session state and is used to log people in and out.