This chapter foreshadows the further 15 chapter contributions in this book, framing them within the evolution and theoretical perspectives that underpin Social Media (SM), and its use in mathematics education. We begin by briefly describing the evolution of SM use, consider the history of these developments in mathematics education, and position this within a range of theoretical perspectives including humans-with-media, social constructivism, personalised learning, and communities of practice. We suggest a classification of SM that distinguishes between the different ways in which people engage with SM, highlight the relevance for educational purposes and the ways in which SM can be used in the classroom, and consider the increasing role of artificial intelligence (AI) in SM. The complex interplay of these many factors for policy makers, teachers and students is connected to the Chapters in this book, drawing out how the chapters collectively contribute to the compelling narrative of the ever-increasing significance of SM for mathematics education, and the ongoing research needed to effectively inform our future use.