At Facebook, we have always been strong advocates of open software. From our earliest days - when the site was built on PHP, MySQL and memcached - we've been privileged to stand on the shoulders of open source giants.

Ever since, we've worked hard to contribute our own work back to the community, and help other companies - both small and large - learn from our experience of building web, mobile, big data, and infrastructure stacks at scale.

Most of our projects are on GitHub, and we also actively contribute elsewhere, such as to the Hadoop projects, LLVM, GNU grep, and Mercurial, amongst many others.

still looking for their post on how it was OpenSource that made them into what they are... I'm pretty sure I've seen one (or was it a video?)