Metadata such as the hashtag is an important dimension of social media communication. Despite its important role in practices such as curating, tagging, and searching content, there has been little research into how meanings are made with social metadata. This book considers how hashtags have expanded their reach from an information-locating resource to an interpersonal resource for coordinating social relationships and expressing solidarity, affinity, and affiliation. It adopts a social semiotic perspective to investigate the communicative functions of hashtags in relation to both language and images. This book is a follow up to Zappavigna's 2012 model of ambient affiliation, providing an extended analytical framework for exploring how affiliation occurs, bond by bond, in online discourse. It focuses in particular on the communing function of hashtags in metacommentary and ridicule, using recent Twitter discourse about US President Donald Trump as a case study. It is essential reading for researchers as well as undergraduates studying social media on any academic course.