Interesting debate but a little flawed on some aspects.
Firstly, this is about web applications which Twitter is (if its not, please explain why not!).
Comparing C++ into the equation is a little short sighted and those people who are defending C++, you really have no argument here as when was the last time someone created an entire web application in C++?
If they did then god help them as its not an area that C++ is designed for. C++ in certain environments, such as technical or scientific areas is the king and there is no doubt about it but it comes down to business requirements.
End of the day as a business you will want to bring an application to market as quickly as possible, hence the need for middle to high level languages, which is where PHP, C#, Java, Ruby and to some extents the more esotoric brands like perl and python fall into. The argument about performance is a bit too vague as again, you have so many factors to consider e.g. is the language in question being tested against the development of an Web Application or a Desktop/Embedded application?
Raw compiling charts don’t mean anything except to the neurotically charged induviduals who care only about these facts.
What has been highlighted is the state of play of the Web Application domian. There are still many areas where Web Application design and development is limited by the technology available however this also offer a unique challenge where the designers have to architect a solution based on the limited resources (including scalability). Developing say a desktop application is not a big deal anymore, end of the day if its far quicker and easier to develop a C#/.NET based application than a C++ equivelant then I am afraid I would hire a C# developer - the average computer is incredibly powerful and as such most coders don’t have to worry about things like performance as they know the system will offer enough bang to cover the limitations.
What is important is how web development proceeds in the future. Its obvious RoR has limitations that Twitter has encountered and it would be good to see the dev’s taking this oppertunity to expand out of the box and develop the design further to handle other database systems and improving the language overall.
But as someone mentioned, most preformance issues in a web environment are to do with networking (internet speeds, hardware), server platforms and the client system that is being served.
We are essentially running systems that are incredibly powerful down a network that is barely capable of handling the demand, until this area improves we will always have issues that are outside of the control of the coding language of choice.