Splitting is the new 'thing', I fear
Facebook aren't the only ones doing this insane "we'll force you to use two apps instead of one" bollocks. FourSquare are removing the option to check in from their app, and trying to persuade users to use Swarm.
Some people may well respond faster when they use Messenger; anyone who tries to send me a message on Facebook now will likely get a slower response, because I uninstalled both apps from my phone, and they'll have to wait until I'm using a browser, and notice there's a new message.
The main app knows you have a message; it knows who it's from, and even at least a bit of the text, so it can pop up a notification that then allows it to say "hahah, gotcha, now install something else to read the message."
Sometimes it seems that the people creating these apps assume that everyone uses them in the way they do; that everyone has the latest devices with lots of storage space. With FourSquare/Swarm I honestly can't see the point for me; I don't give a toss where people are right now, because I'm not a 20-something valley geek who hangs out at just a few places with all my mates, which seems to be the point of the new app. I suspect FS on my phone is likely to go the same way as the Facebook app, because that'll be less frustrating than bouncing between apps to do different functions that all used to be in the same one.
Take two apps into the shower? Sod off with that.