That's assuming you can replicate their build process exactly and that process produces perfectly reproducible builds.

Can you? I was under the impression that perfectly reproducible builds were still very much a hard and open problem.

Furthermore, I believe that the Facebook app codebase is massive in scope and highly illegible due to most of it being auto-generated from other codebases. It has over 18,000 classes on iOS. The odds of anybody being able to meaningfully audit that are pretty low.