In a Q&A session yesterday, the head honcho of Facebook decided to explain the decision behind making messenger a standalone app.

At the time, it was one that annoyed a huge number of users as concerns were raised over the sheer number of permissions that you were handing over to download the app, and what exactly they could be used for, so when the question was put to Zuckerberg in a live Q&A session yesterday evening, he gave an answer. 

As you might expect, it was refreshingly honest, concise, and he thought of it just off the top of his head; it contained no marketing speak and gives the real reasons that they wanted you to have a Facebook app next to your Facebook app on your phone. Ah we're only messing, it's all gibberish from the PR department.

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. 

Essentially: "we did it for you, baby. Everything we ever did was for you". Not sure we're buying it Mark. 

Via The Verge