Internet companies such as Facebook operate in a "perpetual development" mindset. This means that the website continues to undergo development with no predefined final objective, and that new developments are deployed so that users can enjoy them as soon as they're ready. To support this, Facebook uses both technical approaches such as peer review and extensive automated testing, and a culture of personal responsibility.
More than a billion users log in to Facebook at least once a month to connect and share content with each other. Among other activities, these users upload more than 2.5 billion content items daily. Here, we describe the development and deployment of the software that supports this activity, focusing on the site's primary codebase for the Web front end. Information on Facebook's architecture and other software components is available elsewhere.