That is certain. If you are trying to run at a very large scale with a single repo, Perforce (or apparently Mercurial) is the way to go.
The problem is "how do you grow even further?" There isn't a perfect solution to that (yet), but I think the only existing workable (although absolutely imperfect) solution is splitting your codebase into many small repos.
Thankfully very few companies need to worry about this problem right now. A single Mercurial repo is apparently scaling for Facebook currently, and few companies will manage to stress Perforce.
The problem is "how do you grow even further?" There isn't a perfect solution to that (yet), but I think the only existing workable (although absolutely imperfect) solution is splitting your codebase into many small repos.
Thankfully very few companies need to worry about this problem right now. A single Mercurial repo is apparently scaling for Facebook currently, and few companies will manage to stress Perforce.