Social networking sites are a part of everyday life for over a billion people worldwide. They show no sign of declining popularity, with social media use increasing at three times the rate of other internet use. Despite this proliferation, mental health care has yet to embrace this unprecedented resource. We argue that data from social networking sites should become a high priority for psychiatry research and mental health-care delivery. We illustrate our views using the world’s largest social networking site, Facebook, which currently has over 1 billion daily users (one in seven people worldwide). Facebook users can create personal profiles, socialise, express feelings, and share content, which Facebook stores as time-stamped digital records dating back to when the user first joined. Evidence suggests that 92% of adolescents go online daily and disclose considerably more about themselves online than offline. Thus, working with Facebook data could further our understanding of the onset and early years of mental illness, a crucial period of interpersonal development. Furthermore, a diminishing so-called digital divide has allowed for a broader sociodemographic to access Facebook, including homeless youth, young veterans, immigrants, people with mental health problems and seniors, enabling greater contact with traditionally hard-to-reach populations. While acknowledging that issues are far from settled about the role that social media should play in mental health, we argue that it should no longer be a debate about whether researchers and health-care providers engage with social networking sites, but rather how best to use this technology to promote positive change. We discuss how Facebook data can advance psychiatry research and how user-level data could potentially enhance the clinical delivery of personalised patient care. More specifically, we illustrate how Facebook data can assist with identification, intervention, and possibly prediction and prevention of mental illness.