hmm odd, sorry to get your hopes up, I don't know why it suddenly worked for me as I didn't do anything other than update the facebook app although I did notice something that maybe wasn't coincidence.
I have an UK s7 edge and when it failed to work was when I started deleting 360 photos from the Gallery that seems to originate from a Samsung Cloud directory (of which I couldn't find in Es Explorer), as it was showing duplicated photos in that app. It was working prior to this as I had just uploaded a 360 photo to facebook.
Now when it worked for me again (which may/may not be a result of Facebook app update) I noticed in the gallery that photos were appearing to be coming back from Samsung cloud.
Is this something to do with Samsung cloud and the fact it's messing up with the file sharing mechanisms of android and affecting uploading to facebook?
Maybe try dropping back to a phone firmware that didnt include the samsung cloud update and see if that fixed it?
I may try disabling cloud syncing but since it's working now for me I don't want to touch it!