Zuckerberg was asked why Facebook Messenger had to be spun out as a standalone app:
Asking everyone in our community to install a new app is a big ask. I appreciate that that was work and required friction. We wanted to do this because we believe that this is a better experience. Messaging is becoming increasingly important. On mobile, each app can only focus on doing one thing well, we think.
In order to get network effects, Facebook pushed a new app on the community. It was a smart move.