Hi Taha,
From Cinema 4D you need an FBX file, then use a converter to get the file needed for FB.
I saw it a few weeks ago and researched it a little bit. Here are my findings:
Facebook is currently requiring this:
https://developers.facebook.com/docs/sharing/3d-posts/asset-requirements
It seems that they work on it, so it might change all in a short time.
At the moment, all ways point to:
https://github.com/facebookincubator/FBX2glTF
The suggestions I found so far on the web
• Keep the Textures in multiple of 256, as in 1024 pixels wide or high.
• One Material per model.
• All material should have a specific name.
• PNG as file size.
If you would like to have a direct export option, perhaps you suggest this directly to MAXON, thanks for your time.
https://www.maxon.net/en-us/support/suggestions/
BTW, I do not ignore “online” converters, I just DON’T use them, be always careful with un-known web-sites who make that offer. To upload a file might be a very dangerous idea.
All the best