I work at Meta on compiler support for all kinds of apps. We regularly write and use some pretty aggressive compiler optimizations and we often experiment and evolve lots of compiler and programming language features that are really experimental for use in our apps and services.

I don’t think you can directly compare an app like the ones a small team can build to the apps that larger more established companies have to maintain and evolve. I mean sure you could just do a full app rewrite and start from scratch but that’s hugely risky and certainly fails more often than it succeeds.