When working on a mesh, I now am used to upload it on Sketchfab. I often re-upload an updated version of a mesh and then use the "Save view" option to always get a thumbnail that reflects the object as it is.
Pb : when I update a thumbnail in Sketchfab ("Save View") the thumbnail is also updated in my website, which means my website loads the image from your server. But when I open a thread in Facebook and post a sketchfab link, the thumbnail displayed when I first created the post will never change. No matter how many times I update the thumbnail in Sketchfab.
So I guess Facebook is storing their own preview. And never check the SKetchfab server to get the latest version (which is damn dumb). I tried to submit them the issue for months, literally.
As I don't think this issue have nothing to do with you, I was just hoping that maybe you could try to contact Facebook about this problem. A mail coming from Sketchfab.com might have a more serious impact that one coming from me, Mr.Unkown. If it was any other website... but facebook is quiet something.
Thanks for considering that