Craig Buchek

That's a really ambitious project. So you're going to want to move as quickly as possible. That means choosing a software stack that will allow you to move the most quickly. Of the ones you've listed, Ruby (Ruby on Rails, more precisely) will give you that. There are probably other options that would allow you to move even more quickly than Rails, but developers for those more advanced stacks are much more expensive, and hard to find.

Rails can and does scale to millions of users. It's not necessarily easy, but if you're running a successful app, you'll be able to afford people to help you scale out when you reach that point. But if you choose a slower framework (or have bad developers) you won't even reach the point of needing scalability.