If a friend disconnects from FB you won't see them in your friends bar. There was an issue not too long ago where the one active FBer on my game couldn't see me and I couldn't see her for a while (that's been fixed). I had another friend who played Hay Day for a while and connected through FB. She deleted the game (or just disconnected I don't know which)- she disappeared completely.
No idea if change in number is to do with fb people but i would guess yes- especially if they either "disconnect from FB" or delete the game outright severing all contact.

