>Moreover the git maintainers were unwilling to work with Facebook to improve git to solve some of these problems. Mercurial was a better fit

More add-on discussion including Microsoft+Git spurred by your previous comment about this: https://news.ycombinator.com/item?id=15725497

From the Facebook blog post, it seems like the key issue was Facebook's internal filesystem monitoring tool (Watchman) was easier to integrate with Mercurial than with Git:

https://engineering.fb.com/2014/01/07/core-infra/scaling-mer...

So, neither Mercurial-out-of-the-box nor Git-out-of-the-box could handle huge monorepos. But Mercurial's willingness to make some modifications made it easier for Facebook to integrated their custom tooling to avoid the slow Big-O O(n) scans for changed files.