I actually believe part of it is that they rely on this editor[1] for the messenger and anyone who tries to use it on any device that supports IME (like phones) will find that it does not work at all.
Not so much that it doesn't work at all. More like, it appears to work and then fails horribly down the road.
I've been bitten by it too much. It's typically the auto-complete feature of mobile keyboards that fails. Something that you don't always see in emulators.
[1] https://github.com/facebook/draft-js