I've had an estranged family member and a job offer come to me through Facebook messages. They both ended up in the "other senders" section on the website - which I didn't even know existed for years. I read some news story on messages lost into that folder, checked it, and realized that major events in my life had disappeared into someone's shitty UX choice.

I don't even know what to do about this - deleting my FB account might stop people from doing this, but it's far from guaranteed and any test I run now will be invalid as soon as the UX changes again.

I believe that if you don't have the app, Facebook sends you an email saying you have new Messenger message. I've never used Messenger, and this has happened to me.

The problem isn't whether or not you have Messenger; the problem is that (as a spam prevention measure?), FB will blackhole messages sent to you by non-friends.