I take away two lessons from it, which are in a kind of essential tension that can only be mediated by wisdom and experience: 1) Pick technologies that you can

I take away two lessons from it, which are in a kind of essential tension that can only be mediated by wisdom and experience: 1) Pick technologies that you can optimize. 2) Don't over-optimize. Also, the concept of "optimization" here has very little to do with the language itself. It's far more about the overall stack, and it definitely includes people and processes (like hiring). It's not like FB invested $0 toward performance before swapping out PHP interpreters! Its massive caching layer, for example, was already taking shape well before HPHP (the C++ transpiler which preceded HipHop), not to mention the effort and tooling behind the MySQL sharding and multi-region that still exists in some form today. Many backend FB services were already written in C++ by 2010. But they had already gone very, very far—farther than most businesses ever will—on "just" PHP. Heroics like HPHP only happened after enormous pipelines of money were already flowing into the company.