Dan Abramov's comment elsewhere in this thread says otherwise:

"I work at Facebook, and can confirm we keep all code in a monorepo".

https://news.ycombinator.com/item?id=15893184

Unless something drastic changed in the last year, I really doubt it. There is the fb frontend, the backend, the offline batch processing repo, and the instagram frontend repo. I think the phone apps have their own repos too? It was a giant mess, especially when you had to make changes that spanned repos, like introducing a new backend API and then depending on it, or changing logging formats.