I’m probably one of the rare individuals who has yet to actually register for Facebook, and have never actually seen the site, however having read about the sheer number of users and traffic, clearly they’ve managed to build a massive infrastructure. That said, I read with great interest about their release of a code-generation framework known as thrift, which according to the site, builds “services that work efficiently and seamlessly between C++, Java, Python, PHP, and Ruby.”
On April 1 they released the code, and offer a fascinating whitepaper regarding the thinking behind their project, which you can download here (PDF).