I've noticed that sometimes Facebook shows a fake 'youve got a message' icon to try and trick you into installing their messenger app.
To re-produce this behaviour: (this works best if you dont get a lot of facebook messages. Also you need a phone with no facebook apps installed)
- On your desktop PC, use facebook to send a message to someone
- Then switch to your phone, using facebook in the browser
- After about an hour, the little speechbubble message icon at the top will go red, showing you've got a message
- If you click on this from a phone browser, it redirects you to install messenger. (normally, phone-browser facebook doesnt do messenger features)
- Instead of that, switch on 'request desktop site' (not sure what iOS calls this option) to make your phone display the desktop version of facebook. And then you can (usually) read your messages in the browser
- But you will find that there is no new message, and the new message icon will no longer be lit up.
I've had this happen five or six times now - supposedly new messages have arrived but when you look there are none. It always happens about an hour after sending a message. I'm pretty convinced its deliberate behaviour on the phone browser version of facebook to get you to install their messenger app.