This study investigates how emergency response organizations use social media during emergency preparedness and response. Using qualitative (interviews and documents) and quantitative (Facebook posts) data, the study identifies several uses of social media in emergency preparedness and emergency management, as well as the organizational context that affects this use. Findings indicate that social media support various purposes of use, including information dissemination, obtaining input from the public and other organizations, and participation by other emergency response organizations. Branding of the organization during the emergency preparedness phase was found to be an important aspect of information dissemination, and helps social media to be useful tool to connect with the public and other organizations during the emergency response phase. Nevertheless, social media use in the emergency response domain still has to overcome leadership and staff adoption barriers.