Prior to the unbundling, responding to a Facebook Message requires the overhead of loading up the news feed and at least one extra tap to see messages.

I suspect this also has to do with internal development mechanics as well. Unbundling lets the Messenger team push app updates out more quickly, without having to worry if their changes broke the main Facebook mobile app.

Not sure what's up with Paper, but I'd guess it's about letting the Paper team experiment with new UX ideas without fear of upsetting the majority of users still using the current app.

I don't think the unbundling is succeeding (or will succeed). I like them all in one place, though fb mgmt seems to have convinced itself that unbundling is a better user experience somehow..