On VentureBeat today there's a post (the first of two parts) looking at Facebook's involvement in Open Source software, including their work on HipHop and XHP for PHP (an interview with David Recordon and Amir Michael).

The social media company has, without question, taken the time to work on those projects. The hackers at Facebook have done perhaps more than any other single entity to advance and optimize PHP, the programming language on which the network is primarily built.

They talk about some of the projects, both PHP-related and not, that the developers of Facebook have either created or contributed to. There's mentions of Facebook's "hacker culture" and a mention of their workflow and how they decided on going forward with something like HipHop.