Students today spend more time writing on social media than they do in formal classrooms and have developed distinct and complex literacies separate from traditional writing curricula. Although several studies have explored teens' informal literacy practices online, research investigating how these spaces may inform or be effectively utilized in the classroom remains inconclusive. This systematic literature review identifies and critically evaluates the existing knowledge base on how social media have been integrated in high school writing instruction, with particular emphasis on which factors improved student learning and which factors created barriers to student learning. Social media platforms were found to be potentially authentic learning spaces, yet sometimes low student participation hindered effective implementation. Specific teaching strategies, such as modeling and scaffolding instruction, and directions for future research are described.