My advice here is not to plan for premature optimizing and scaling. Between Python, NodeJS and Erlang you have three options to choose from. However, trying to choose between them should basically come down to your personal comfort level and knowledge with each language.

I am just assuming here - but you will probably not reach 500k users overnight, so building on something which can allow you to maintain a good solid codebase would be more important.

NodeJS is still not stable (relatively speaking to Python), but from my own experience and what I have read, it does scale well. What you have to be wary about is some frequent changes to the API etc.