I think this happens when certain ads come up in the news feed. All's well until you scroll past them, and then the app crashes. This happened to me a few months ago, and again today.
Facebook has been the worst behaving app for me on iOS 8. It lags a LOT, and this crashing issue has now come back.
You'd have thought Facebook would get early access to the 6/6 Plus and get priority updates pushed through the app validation.
Twitter managed it with their piece of **** app, why can't Facebook?