[Q] Keep facebook session between app sessions topic

I know it should go to the software development but I don't have access rights so I got a message to post my question here, I hope this is the right place.

I know how to login to Facebook and use its info but I don't understand how to keep this session alive if the user presses the home button and opens my app again. There are a lot of functions related to session like getActiveSession, openSessionFromCache etc. but that's only making me more confused.

I have one activity that holds the different fragments when they're needed so I guess I should do something in MainActivity's onResume but I don't understand how can I check for Facebook session existence and if it is, use it or "wake it up" and continue from where the user has left it.

I searched the net but I keep getting to posts about logging in to Facebook but not how to maintain it afterwards
Please advice
Thanks