I am looking at Facebook Android SDK examples, and I have encountered this strange pattern (in Scrumptious example): all (or almost) the screens of the app are fragments, loaded into the application at the beginning but hidden.
Later (resume, Session updates, eccetera) one fragment at time is showed, while all the others remain hidden.
This way there is just 1 activity and all the application screens are present at the same time on the activity, just hidden rather than displayed.
Is this some kind of design pattern for Android? Pros & Cons?
I wouldn't call that a design pattern, but yes - this is quite common.
The main issue with layout inflating is that it may take a lot of time. Keeping the screens prepared reduces the time needed to switch them. It also eliminates the animation hiccup - the main user experience killer. With Material Design in mind this becomes quite important.
The obvious problem with such approach is the memory usage. Facebook displays a lot of data as lists, so I guess they clear all adapters except the one visible on the screen.