This chapter focuses on a set of actors who played a crucial role in the domestication of social media by marketing. It presents an exploration of the business of social media marketing agencies. The domestication of word of mouth on social media sites by marketing is not self-evident. The chapter discusses the adventure of social media marketing is – or at least it is when one adopts the perspective of professionals – the history of the domestication of marketing by marketing, rather than the history of the domestication of word of mouth by marketing. The emerging market for social media marketing services is thus organised into three specialties: contagion, influence and community. Contagion is the figure of social relationships that underlies the area of social media marketing (SMM) agencies. This figure of the social relationship has led to the emergence of a large body of literature in social science, which has later been translated into the science of marketing. [Premier paragraphe]