This is an interesting article about Twitter's shrinking use of Ruby and Rails. (My friend and coworker Abdul Habra sent it my way.)
The upshot is that Ruby and Rails are better tools for rapid development with a small team, but that they don't compare to the JVM for encapsulating things so that multiple teams can change things at the same time. There is also a performance gain, but that's a secondary concern.
Twitter seems to have taken a very smart route here - build their product quickly with "Agile" tools, then change as needed to support a larger user base.
I'm particularly geeked (pun intended) about their choice of JVM language. They went more cutting-edge with Scala as their language of choice for most applications.