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.