Same issue here, still a problem as of ver. 1.1.8, not sure what caused these fields to disappear. I have no mentions, and no custom status to page fields when creating new posts. I DO have the custom status to timeline field, but that's it. The mentions that I used in a previous post still show on said post, but the option to add mentions to posts has completely disappeared.
I tried the above mentioned troubleshooting options to no avail. Since there aren't a ton of people crying foul on this issue, I'm assuming many of us have something in common that may be causing this issue. The only plugin I have in common with one of the above posters is NextGen - maybe there is some setting missing on our FB apps...I dunno.
my site is silenciobarnes.com
It looks like the guys over on this thread are having the same issue: https://wordpress.org/support/topic/mentions-missing