Facebook is a fan of open source software. We are heavy users of packages like PHP, MySQL, Memcache and also believe in contributing technology like Thrift and Scribe back to the community. It's interesting not just to hear that companies are using open source software (everyone is!) but rather about how they adapt the software to better suit their needs.
The video and slides linked to below provide details on our particular usage patterns as well as the various optimizations and modifications we have made over the years. In addition to the core web-tier stack, we also have a number of custom backend services that power products like NewsFeed and Search. The slides give an overview of the services (!php) stack and the various toolsets that allow Facebook engineers to easily develop, deploy, monitor and maintain code written in a variety of languages like C++ and Erlang.
Slides: QCon Slides
Aditya is a Facebook Engineering Director working on NewsFeed, Search and various components of the services stack.