Islamic State (IS) terrorist networks in Syria and Iraq pose threats to national security. IS' exploitation of social media and digital strategy plays a key role in its global dissemination of propaganda, radicalization, and recruitment. However, systematic research on Islamic terrorist communication via social media is limited. Our research investigates the question: How do IS members/supporters use Twitter for terrorism communication: propaganda, radicalization, and recruitment? Theoretically, we drew on microeconomic network theories to develop a theoretical framework for multi-sided Twitter networks in the global Islamic terrorist communication environment. Empirically, we collected 3,039 tweets posted by @shamiwitness who was identified in prior research as "an information disseminator" for the IS cause. Methodologically, we performed social network analysis, trend and content analyses of the tweet data. We find strong evidence for Shamiwitness-intermediated multi-sided Twitter networks of international mass media, regional Arabic mass media, IS fighters, and IS sympathizers, supporting the framework's utility.