This thesis explores how participatory online rituals of mourning serve to mediate public death events that are collectively experienced as forms of social injustice, and the modes of collectivity they engender. I introduce the term Social Media Memorialising (SMM) to describe this phenomenon. The mediated deaths of SMM are experienced as a transgression of the sacred, and in the process reveal societies' constant negotiation with death, virtuality and memorialising online. SMM entails appropriating the processes of public mourning such that the means of symbolic production shifts away from media and political gatekeepers and towards networked publics. In analysing SMM on YouTube, this thesis employs a mixed-methods research design premised upon a multimodal approach to discourse, system-network mapping, and thematic analysis. I present two case studies for comparative analysis: those of Neda Agha-Soltan in Tehran in 2009, and that of Lee Rigby in London in 2013. Both constitute emblematic examples of 'public death events': the death of individuals considered to be exceptional, morally significant, traumatic and worthy of public mourning and grief. This framework captures the complex forces involved in the mediation of death online, and the modalities and mechanisms of virtual space as ritual space. SMM manifests through innovative, strategic and performative forms of grieving that hybridise online and offline practices, highlighting the conditions of the death event as integral to the modes of grieving that follow. What emerges is a platform-specific vernacular that reflects the form, function and terms of engagement for online grieving. SMM coalesces the commemorative with the performative, shaping both the social significance of the death event and the attitudes regarding the death and its causes.