Surprisingly (or rather not), Facebook’s glTF viewer appears to be Three.js-based…
Are you sure about that? One of the devs (I think) said otherwise on GitHub the other day.
EDIT: this comment: https://github.com/mrdoob/three.js/issues/13397#issuecomment-368894618
as far as I can see from the browser console, FB uses vanilla Three.js
Wow, cool! I guess they are using their own loader and three.js for rendering.
Seeing logs from THREE.GLTFLoader as well, although they must be using a forked version as r88 didn’t support the unlit extension…
Interesting… Google Poly is also based on Three.js/glTF even if the source files must be in the OBJ format.