Aside from a few specific edge cases, any limits on scaling you hit when using any technology—Rails included—will be down to application architecture rather tha

Aside from a few specific edge cases, any limits on scaling you hit when using any technology—Rails included—will be down to application architecture rather than the performance of the framework or language. In particular, I don't think comparisons to Facebook or Twitter are particularly helpful. Twitter is basically global telecom-scale; Rails is never going to be the right tool for that, but the flipped of that is it was possible to get to that point using it. Basically, use whatever framework or language that suits your other requirements. Build a sensible, modular system using standard software engineering principles. Don't sweat over the scalability of your framework until you hit scale, and you'll have a great problem to deal with.