[KeytarHero]

Yeah, this is my theory too. I use Tinfoil for Facebook on Android, and it shows I have 15 unread messages. Basically, every time I open Tinfoil while I have a new message I haven't read in Messenger, it increments, and that never gets cleared. If they were trying to drive engagement or to get you to install Messenger, why would they want the number to keep increasing well after you've caught onto the trick?

[r3bl]

Did you add any friends lately? Do you know those "greetings" that Facebook pushes into Messenger whenever you add a new friend on Facebook?

I've recently noticed on my SO's phone that Messenger counts them as "unread messages", and the only way to clear them is to a) send that greeting, or b) clear them from a desktop.

In other words, they're getting pretty desperate to drive the engagements up, and this is something that probably wasn't implemented in third-party clients.

Note that this is different than a "message" on a mobile browser. Last time I've checked (admittedly, quite some time ago), there was one "message" that never gets cleared, regardless of what you do.