Saying “Rails doesn’t scale” is like saying “my car doesn’t go infinitely fast”. Alternatively, saying “We’ll have no problems scaling because we’re using Django” is like saying “I will win every race because my car is the most powerful”. Maybe so, but you suck at driving, and you’re up against professionals.
If you’re having scalability problems and blaming it on a single technology, chances are, you’re doing it wrong.
Ted Dziuba on Scaling