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.
[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...