Abstract Facebook is the world’s largest social network, connecting over 800 million users worldwide. The type of phenomenal growth experienced by Facebook in a short time is rare for any technology company. As the Facebook user base approaches the 1 billion mark, a number of exciting opportunities await the world of social networking and the future of the web. We present a case study of what it is like to design for a billion users at Facebook from the perspective of designers, engineers, managers, user experience researchers, and other stakeholders at the company. Our case study illustrates various complexities and tradeoffs in design through a Human-Computer Interaction (HCI) lens and highlights implications for tackling the challenges through research and practice.