This is impossible to answer without knowing more about your app. For web apps, I am definitely biased towards productive environments such as Rails (in the Ruby world) or Django (in the Python world), over something like a more heavyweight J2EE stack. But how your app will scale will depend on a lot more than just your choice of web framework. For many social applications (such as Twitter), the website ends up only being a relatively small part of the overall system, and how the rest of the system is architected becomes very important. Is your app componentized in a way that allows you to scale up individual components as demand increases? Are you going to be able to cache the hell out your system to avoid database access? Etc.

Anyway, good luck with your app!

Would you suggest we do NOT use Ruby on Rails?