Why do you think this is madness? FB doesn't use git for these monorepos, so that's not really relevant, but I don't understand why you think that it's better to break a repo up because the SCM can't handle the size vs fixing the SCM so it can handle the size. I work for meta (nothing to do with the teams working on this though) and I can assure you people have considered the tradeoffs of breaking repos up just to accommodate existing SCMs vs improving the SCMs.. I think if you believe improving the SCM instead of breaking up the code is madness you should probably provide a bit more of a justification

You skipped a step. They actively contributed to mercurial before the full fork.