Positing online users as 'sleepwalkers', Tony Sampson offers an original and compelling approach for understanding how social media platforms produce subjectivities. Drawing on a wide range of theorists, including A.N. Whitehead and Gabriel Tarde, he provides tools to track his sleepwalker through the 'dark refrain of social media': a refrain that spreads through viral platform architectures with a staccato-like repetition of shock events, rumours, conspiracy, misinformation, big lies, search engine weaponization, data voids, populist strongmen, immune system failures, and far-right hate speech. Sampson's sleepwalker is not a pre-programmed smartphone junkie, but a conceptual personae intended to dodge capture by data doubles and lookalikes. Sleepwalkers are neither asleep nor wide awake; they are a liminal experimentation in collective mimicry and self-other relationality. Their purpose is to stir up a new kind of community that emerges from the potentialities of revolutionary contagion. At a time in which social media is influencing more people than ever, A Sleepwalker's Guide to Social Media is an important reference for students and scholars of media theory, digital media and social media.