The Verge has an article: Mark Zuckerberg finally explains why he forced you to download the standalone Messenger app.
The need to install Messenger as a separate app (after months and months of ignoring the main FB app's pleas to, with promise of how "fast" it was) irked me mightily. I consider smartphone front page app space valuable - but I am interested enough in getting notified about activity on FB (whether post comments or messenger, the difference isn't all that much to me, really) that I had to carve out two places, since I would miss the notification badge on a secondary page. (Later I came to split the difference: the only app folder I have on my iPhone's first page is now "social media" in general, including Messenger, some dating sites, and some things for work I usually use on the laptop, while FB's main app gets to sit on the page itself.)
Anyway, for me the heart of his explanation was:
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.I am messaging people 15 times a day. Or at least 4 or 5. But it's the "But not on FB" that's Zuck's problem, the sentence he leaves out at the end of that. He'd love FB to be as critical for people in the quick messaging sense as it is in FB's traditional niche.
I don't know what FB's IM numbers are, but if the usage by me and my friends is anything to go on, the IM there is secondary. You use it when you aren't looking for a quick response, and you use it because if someone is on FB, they have access to a keyboard, and it's probably more physically convenient to tap out a response. (I know I'm talking like an old guy, where the web is more my native communication form than mobile.)
But FB is the big dog that sees this bone some other dogs are fighting over (Apple's iMessage and WhatsApp in particular) that, despite the delicious bone it has on its own plate, it wants to go and get a piece of that, no matter how many user's toes they step on. They want to own quick communication as well as the longer term "oh yeah that jerk I knew in middle school" longer term stuff.
FB, Tumbler, and Twitter collectively sucked a lot of the air from the independent web of the mid 2000s, because their value proposition of "build a centralized collection of possibly interesting stuff from people you know and/or expect to make or find cool stuff" is strong; it beats having to go from site to site, and hardly anyone bothered with RSS feeds for that (which I tend not to like, because it takes away the UX flavor I find critical in knowing where I saw something...)