Facebook Login contains the neccessary documentation. Manually Build a Login Flow describes the flow used by the plugin in greater detail.

Long-lived tokens

Facebook offers the option to switch a normal access_token for a Long-lived Access Token. This token lasts for about 60 days (the normal one lasts around 1 day by default), therefore this might be a good way to not have the user perform a re-log every time they open the game.