What kind of messaging architectures are used in huge, scalable sites today?

Sites like Twitter and Facebook scale to hundreds of thousands of users. Most of their architectural overviews are available online as talks and slideshows. However, my question is more oriented towards any messaging middleware/layer that these sites use. I understand that it would be different for differe开发者_StackOverflow中文版nt sites - but are there any common characteristics when using messaging technologies (e.g. JMS) on highly scaled sites? More specifically, are there use cases that cannot be handled by traditional messaging solutions?

Twitter switched to Scala for their middleware and got huge performance boost. High-scalability is the best authority on the topic of scaling web applications.