I don't think you fully understand how big their codebase is and how many different teams are working on it at any given time.
There are very fundamental differences between a mono-repo and a bunch of repos for each "service" or whatever. Lots of tradeoffs. I've worked both and I can see the reasons for huge monorepos. They make a lot of things that were previously hard much simpler... The tradeoff is your tooling needs to be able to scale with growth of the company. And for a company the size of FB, dedicating an entire team to improving the tooling for their monorepo is well worth it.
Saying that Meta "reinvented CVS" means you either know very little about source control or you are just being purposely misleading. Neither is a very good look.
There are very fundamental differences between a mono-repo and a bunch of repos for each "service" or whatever. Lots of tradeoffs. I've worked both and I can see the reasons for huge monorepos. They make a lot of things that were previously hard much simpler... The tradeoff is your tooling needs to be able to scale with growth of the company. And for a company the size of FB, dedicating an entire team to improving the tooling for their monorepo is well worth it.