I'm having trouble understanding how one could run into a bottleneck only after you reach Facebook or Google or levels. Surely if you have performance issues when you are running on 10,000 machines, you would also have a bottleneck when you start to outgrow your single machine.
This is just wrong. Rails works very well for 90% of the situations. When it stops working that’s great news:
You’ve won!
You’re Twitter, Facebook or the next big thing. You’ll find talent and money to replace your rails app.