In a live Q&A session today, Mark Zuckerberg finally told us why Facebook moved messaging out of its main app and into a separate, standalone app you have to download. His answer:

I'm grateful for hard questions. It keeps us honest. We need to be able to explain clearly why what we're thinking is good. 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.

The primary purpose of the Facebook app is News Feed. Messaging was this behavior people were doing more and more. 10 billion messages are sent per day, but in order to get to it you had to wait for the app to load and go to a separate tab. We saw that the top messaging apps people were using were their own app. These apps that are fast and just focused on messaging. You're probably messaging people 15 times per day. Having to go into an app and take a bunch of steps to get to messaging is a lot of friction.

Messaging is one of the few things people do more than social networking. In some countries 85 percent of people are on Facebook, but 95 percent of people use SMS or messaging. Asking folks to install another app is a short term painful thing, but if we wanted to focus on serving this [use case] well, we had to build a dedicated and focused experience. We build for the whole community. Why wouldn't we let people choose to install the app on their own at their own pace? The reason is that what we're trying to do is build a service that's good for everyone. Because Messenger is faster and more focused, if you're using it, you respond to messages faster, we've found. If your friends are slower to respond, we might not have been able to meet up.

This is some of the hardest stuff we do, is making these choices. We realize that we have a lot to earn in terms of trust and proving that this standalone messenger experience will be really good. We have some of our most talented people working on this.