Hello
Sometime i don't have any preview picture on facebook shared model.
I do have a preview picture on sketchfab but when i choose "share" the preview picture is not there...
like on my last model : https://sketchfab.com/models/9010a27212e74181a6d8a94e575eca99
Sound like an issue on Facebook's end.
If you copy and past the model URL into a Facebook post, it should appear ok. Sometimes while you're writing the post, it doesn't appear correctly, but it should after you actually submit the post.
I had tested this option to copy + paste the link directly on my facebook statue ...but it's the same ....
I have upload the model again and now it work....
Maybe be i will delete now the not working one ...
Thanks for the answer anyway 
Also, Facebook caches all our data. So if your model was set to private when you first posted a link, it wouldn't be able to fetch it, and it will never become visible on Facebook even if you make the model public on Sketchfab. In such cases, a fresh upload is the best solution.
I have got this problem.. but I have two facebook accounts, and tried to embed the same private model with password to those accounts.
One of the account does show the thumbnail of the model. Another does not.
StudyRoomPractice by Yaldroid on Sketchfab
... how can I make its thumbnail visible to another account without fresh upload? 
thank you, best regards.
