Thank you for your interest in sharing your idea with Samsung. We appreciate your feedback and will tag it on the forum so that it will appear with other feedback, ideas, and requests. This will allow your post to be more easily found in the event that Samsung product groups are seeking outside feedback/requests/ideas regarding our products.
I believe facebook messenger isn't supported because it opts to have it's own notification settings outside of the operating system. This isn't something on Samsung's end but Facebook's. At least, from how I understand it.