Earlier quoted context omitted.
Isn't this more of a reflection of when these companies originally developed their platforms? Similar to Facebook and their use of PHP. It would be interesting to see in a few years what the next successful unicorns are building with today.
Anecdotally, TypeScript seems very popular among startups today. And for good reason - between Node.js servers, ReactNative mobile apps and React web apps, you can build everything in a single language. A language that’s naturally very productive while getting a product off the ground, pretty maintainable over time, with good enough performance (and concurrency support) for most use cases. Agreed that PHP was the #1…
Performance is really not an issue unless you're doing very CPU-intensive work which you can't offload to a native library for some reason, or you need to scale vertically rather than horizontally (which is even rarer). Compute cost only really starts being a worthwhile optimization target at the "hundreds of billions of requests per day" level, and even then you're not gonna see much of an improvement from moving to Java unless you're actually very good at tuning threadpools (and it might still not be enough of a win compared to whatever else your developers could be doing with that time).