Unless you raise a bunch of capital affording you the time to try and build a scalable architecture before flipping the switch, it's just not feasible to spend time on problems that don't exist (and may never if you're site doesn't take off).
The irony is even if you did try and build an architecture that scales from day 1, there's a really good chance that you'll have to redo it anyway once you spot the real bottlenecks.
System architecture is algorithmnic in nature. There's no charge for having good architecture. You just need to put some thought in up front. In choosing to use Rails - and note this is NOT a knock on Rails - Twitter chose to side-step a large chunk of architectural work - and that choice - NOT the choice of Rails - is what bit them.
Unless you raise a bunch of capital affording you the time to try and build a scalable architecture before flipping the switch, it's just not feasible to spend time on problems that don't exist (and may never if you're site doesn't take off).
The irony is even if you did try and build an architecture that scales from day 1, there's a really good chance that you'll have to redo it anyway once you spot the real bottlenecks.