I've come to realize that Facebook is sort of a global operating system for communities. Your Windows installation includes all the pieces to connect to an Active Directory enterprise network, even if all you'll ever do is play games from Steam. Facebook is like that: there's so much stuff you'll never see when you only use it to follow a bunch of old college friends.
Should those things be downloaded as needed? Maybe. But it would have a negative impact on metrics: if "New Feature X for Latin American Teenagers" takes 30 seconds to start up, it's not going to take off. It's easier when everything is already there. Besides, the effort of rearchitecting a 491MB app would be crazy. Of course Facebook has major public initiatives like React Native which could eventually allow parts of the native apps to be dynamically downloaded more easily (I don't know anything about whether that's planned, just stating the obvious).