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.

Sit down. I worked at FB.

One year in 2014/2015?

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.