Not at all scientific, but almost everybody I know uses an iPhone, and they all use the folders functionality, even the least tech savvy ones, mostly for hiding the unused Apple apps that can't be deleted. My guess would be this is very common behavior. Even if folders aren't used, users might move all the Facebook apps to be together on a secondary screen.
I think we can agree that most users won't fill their home screen with multiple Facebook apps. If they are moving the secondary FB apps somewhere else, be it a folder or another screen, there's a chance they might move the main FB app too.