They keep almost everything in the same monorepo: every service, Facebook itself, the backend tools, many of the 'spinoff' experiences, the user data processing pipeline, the list goes on. All in one spot. Now, imagine 60-70k engineers, all wanting to have that sweet, sweet impact needed for good ratings and promotions. Add to that the fact that LOC and reviews are used as a metric for productivity.
Windows has to eventually compile into a collection of binaries delivered (at least in part) to their various platforms. Facebook/Meta does not have that kind of limitation; as long as each piece can be extracted for its own purposes and deployed, everyone is 'happy'.
Windows is estimated to be about 50M lines of code. Meta, in one post, simply suggests that it is "many times larger" than the Linux Kernel at 17M. [1]
Is it all needed? Probably not. But it certainly exists.
[1] https://engineering.fb.com/2014/01/07/core-infra/scaling-mer...
I’m not personally dealing with repos of this size, but it sounds like this is a problem of the past, thanks to Microsoft. I have a hard time believing Facebook would be a bigger beast than Windows.
[1] https://devblogs.microsoft.com/bharry/scaling-git-and-some-b...