How does social media promote citizen participation in disaster events? By studying the three disaster information websites set up by volunteers during the aftermath of Typhoon Morakot in 2009, I investigate how the online community utilize communication technologies to exchange information and mobilize the masses and how different types of online communities perform gatewatching to ensure the accuracy of disaster information in a short time frame. Finally, in light of these case studies, I revisit issues concerning the taxonomy of online communities.