In this article, we provide an overview of the ways in which disaster managers in eight European countries use social media to mitigate people's vulnerability to hazards. Our document analysis and 95 expert interviews in Germany, Italy, Belgium, Sweden, Hungary, Finland, Norway, and Estonia revealed six distinct institutional social media practices that may reduce disaster vulnerability: sharing educational guidelines, informing and warning the public, identifying citizens' concerns, identifying missing persons, sharing guidelines during disaster, and organizing volunteers. We discuss how these practices could affect people's ability to access, understand, and react adequately to information about risks and hazards. Our findings can be used to improve guidelines for official crisis communication on social media and demonstrate the value of using social media in disaster risk reduction.