Imagine a venn diagram, where the left circle is "FB users" and the right is "Messanger users". I think it's a quite big overlap. Are you seriously suggesting it was a right decision to degrade the service for the users using both services? That's not optimizing, it's just plain stupid. Nothing would have prevented them from both a) letting people do simple messaging inside the FB app, while at the same time b) providing a kick ass advanced separate Messanger app for the users only interested in messaging.

How do you have better data than Facebook on how many users use the Messenger functionality and how many use all the others (feed, etc.)?

And having the same functionality in two apps is not a good idea in any scenario.

I thought it was a technical reason, like there was only so much you could do with an ios app so they needed to turn it into two. I don't develop ios so it wasn't clear to me exactly what the issue was.