I'm also in the "Messenger-only" camp aside from college events before I graduated. Messenger is honestly handy and fun but most importantly for me everyone is on it and active. It's simply the best way to reach someone for me.

The simple reason I do all the communication I can on FBM is that I know it won't get lost in some spam filter.

Perhaps a more nuanced reason is that you don't know it got lost in some spam filter. Out of sight, out of mind...

[1] https://consumerist.com/2012/01/04/how-facebooks-message-spa...

[2] http://www.askdavetaylor.com/how-to-check-facebook-spam-emai...

[3] http://www.cosmopolitan.co.uk/entertainment/news/a42460/anot...

[4] http://uk.businessinsider.com/facebook-filtered-message-requ...

I came across these articles a while ago and decided to check my filtered messages just out of curiosity. I hardly ever use fb anymore so I didn't expect to find anything. Lo and behold though, there was a two year old message from my best friend from elementary school who I had lost touch with forever ago. Lesson learned: don't trust Facebook with anything important, they'll fuck it up even if it's one of their core features.

It seems to have become even more cumbersome and confusing in the recent times. FB does not indicate that there may be message requests or filtered messages (even from "friends") waiting to be examined. It seems pointless when you see a message very late and wonder how it could even be missed. FB's message system cannot be trusted with the lack of notifications about these.

hopefully you have control over the data Messenger (and every other app) with something like Xprivacy.

There's a cost to using the free Messenger service [1]. They also want to make it into WeChat.