When you say Arc, are you meaning to say that this site is written in Paul Graham's attempt at a new lisp?
How did you decide to move to Scala / Node.js in particular? I know the site isn't open-source, but if you guys have published any architecture documentation or other information about what's going on behind the scenes, I'd be curious to see it. I have a professional background in scaling large online services and I'm always interested in seeing how other people solve the problems that inevitably come up.
I am curious what would drive the choice of that language. When it was first released, and pg had built it up as something "for the next hundred years" — it was missing some basic things (like unicode support). I do not know if that has changed, or even if the language has improved since then.
Modern Fortran has some good things in it :-).