We really didn't make the choice to prepare for millions of users. We like having complete control over our code, and nodejs+express gives us that. I'm sure we make some mistakes Rails would correct for us and we take a bit more time on development, but we also learn a lot.
That's a tradeoff, and overall we're happy with it.
Without all that callback spaghetti, how are you supposed to scale to roflmillions of users?