The February 2018 Parkland school shooting quickly initiated passionate social media response on Twitter and Instagram. Research on the effect of large-scale mass shootings, particularly on social media dialogue, is lacking, at a time when emerging research suggests potential for both driving mass contagion and enhancing risk perceptions for public interest communications. This study examines response to the shooting through a content analysis of Instagram and Twitter. Findings revealed that gun violence advocacy and risk perception variables were present more frequently on Instagram, while Social Ecological Model policy-level factors were observed more frequently on Twitter. Advocacy drove engagement on both platforms; however, anger was associated with higher Instagram engagement. Details of gun violence, associated with potential for mass contagion, drove engagement on both platforms.