Social media have great potential for effectively communicating about public health risks so people make healthier and safer choices. If used appropriately, social media can strengthen trust between the public and the institution. If used inappropriately, it may also create distrust. This note addresses some of the ethical challenges in using social media for communication and research. It reflects on opportunities in social media risk communication based on experience from the World Health Organization (WHO) and suggests a code of engagement be included in corporate social media policies that contain guidance as to what conduct is or is not appropriate with a view to maintaining public trust in the institution. The note concludes with considerations about the ethical use of social media in research, which is particularly relevant for entities communicating about ionizing radiation, including during emergency situations.