Ask HN: If FB/Tumblr were developed today, what programming lang should be used?

If Facebook and/or Tumblr were to be developed today for the first time (both predominantly PHP/HHVM), what language should be chosen and why (and if it's a framework, which)?

Python seems to be the go to response, but given the recent performance increases and reduced memory usage of Hack/HHVM - I'm not certain Python is such a clear cut answer.

Then theirs frameworks like Rails, but having a long running process scale to the concurrency levels of FB - doesn't seem practical.

Node.js?

What about languages like Lua/LuaJIT/OpenResty, or Erlang Web?