One Line: It seems some of your activity variables were freed from memory as Android OS needed memory for the Facebook application.
Explanation: When an application in foreground needs more memory than that is available, Android frees some memory from the apps that are running in background. Foreground tasks always have higher priority than background applications.
So, what might have happened to your application while it was in background is that some of its variables have lost their values which you are using in your onResume(). Because of this they are holding wrong values or default values (you can check by using Sysout) as they are re-created when you again bring your app to foreground and due to which some of your code is not working right.