It is probably your RAM memory getting overloaded by the sheer visual content of FB. If I ever have issues on my 3, it is with FB(app or Safari based) and having too many apps open at the same time, using too much memory. What can help is to double click home to open the system tray, long hold your finger down on an icon(any one is fine) until they start to wiggle with negative signs on them, then close them by touching the negative sign. This will free up active memory for you to utilize for what ever app is on your screen & using. Then reopen either safari or your FB app and go that route.
You will still have crashes, just not as many or as often, if you are mindful of your system tray apps & closing them frequently.