VERY difficult to pick a favourite....

I think it's a Facebook image issue.
I think so, too.

It seems they change the links from time to time to discourage people from using FB as a picture hosting service and hotlink from there. I think it happens to all images hotlinked from FB sooner or later. You can post a permalink to the pics, but that takes you that particular post on FB. Anything to make people visit FB.

I don't know how long it actually takes for the URL to expire, but it's quite quick, I think. I hotlinked to a pic on FB in the Inspiration thread of this GB four weeks ago and the link is already dead. I've linked to the new URL of the image and posted a link to the post as well.