Twitter's hashtag categorizations and trending topics offer accessible tools through which people communicate and self‐organize during disasters. This article examines Twitter communication networks, specifically the roles played by nonprofit and government agencies, that were engaged during four disasters: (1) The Boston Marathon bombing, (2) The West, Texas fertilizer plant explosion, (3) The Midwest spring flooding in Peoria, Illinois, and (4) The Moore, Oklahoma tornado. NodeXL and UCINET were used to collect and analyze data and determine network composition and structure. Nonprofit and government Twitter pages provided additional data. Findings indicate varied levels of engagement. While several state‐level public agencies and regional nonprofits used Twitter with some regularity, many failed to participate in hashtag networks that penetrated disparate user groups. The article concludes with a discussion on how organizations might more effectively promote response efforts through this medium.