The changing media environment has led to fragmentation of both media and audiences and thus to new flows of news items across channels and time. It has become challenging to clearly distinguish public and media agendas and their relationships. The traditional way of counting single news items to determine an issue’s importance does not apply online. Articles are not scheduled into fixed publication cycles, they are moved, updated, or shared. Moreover, the variety of channels through which users encounter news loosens the connection between news items and their original source. This paper suggests “diffusion” as a concept to identify agenda-setting effects. News items are assigned different degrees of relevance by users clicking, liking, or sharing it. In turn, such relevance can affect others in assigning prominence to news items. The diffusion process results in a shared agenda which is constantly modified and updated by the entire community of online actors.