I'm not convinced that HipHop is an example of that. As I understand it, HipHop allowed Facebook to increase performance hugely with a very modest investment, and very low risk. Rewriting all the critical bits of Facebook in a different language to realize the same speed-up would likely have required a lot more resources and been orders of magnitude more risky.

The difficulty in "rewriting [critical, or otherwise] bits of Facebook" is probably related to the same decisions that make "single repo for everything" the logical choice for the site.