But even then, as you mention, Git doesn't scale well for them. This is why Facebook is using Mercurial. This is why the Chromium and Android projects are spread out over hundreds of Git repos. This is why Google's internal monorepo is using a Perforce-like homegrown tool backed by Google's distributed wizardry.
Git will start to see significant scaling problems beyond say 1 GB total data, 100,000 files, and X number of refs (there has been significant upstream work around scaling refs - a bunch from Twitter I believe - and I haven't measured how well newer versions handle thousands of refs).
The good news is that unless you are a software project with hundreds of developers, have large binary assets, or are doing something like Twitter and have all your developers push all their in-progress feature branches to the monorepo (madness if you ask me), a Git monorepo will scale for you. A Mercurial monorepo will scale even better. Keep in mind that most people aren't {Twitter, Facebook, Google, Mozilla}, so chances are you won't ever reach these scaling limits.
I don't know what it's like now, but at the time Google was on svn, and there had to be a specially hacked version of it. It used a home-grown internal requirements system, to pull down the tree in a sparse way. And to accomplish that you had to re-specify all your dependencies in these special text files (because specifying deps in two places is obviously better.) And then the tool would spend a lifetime figuring it all out, and then you would still download like 35% of the repo anyway, because the nature of giant repos is to get tangled together.
With languages like PHP it might not be so bad, as you typically are editing one file at a time. But most of Google is Java, and most people like using IDEs for that, so it was a disaster. An IDE likes to touch everything all the time, to re-index and re-compile. And, fun fact: at the time Google recommended you store your entire home directory on an NFS network drive, for security. Everyone quietly ignored that.
It was better in one way: everyone could see everyone else's source, re-using libraries was more common, and branching and contributing patches was more common. It prevented people from hoarding their source code, and just throwing .jars or .so's over the wall. But I think the Github era has shown that this did not require everything being in one giant repo! I mean, at the time, Google had Codesearch and other great tools; even though git wasn't super common at the time, this could have been solved in other ways.
My current company is in the process of breaking up their monorepo, even though our codebase is nowhere near the same size. Lately I get to taunt my co-workers because I created a new project in the new broken-up way. All the tests on my micro-repo pass in 5 seconds, and it takes many minutes for the big one.
The list of users is value free, it is just a mention of companies I know using it.
Both the Google and FB talks about this mention the technical challenges, so i guess this is really a downside at very big scales.
They have a single git repo with way more files than that tool was ever designed to maintain. As such, normally instantaneous commands like status, log, ls, etc. take an inordinately long amount of time to execute. Apparently their onboarding process is "have someone copy his working copy to a flash drive, then copy it from there to your laptop" because git pull would take too long.