By integrating Media Richness Theory, Channel Expansion Theory, and the Technology Acceptance Model (TAM), we study the postadoption use behavior of instant messaging. We developed the construct “use richness” as a measure of the extent to which users use the media communication capacity after adoption and proposed a conceptual model of the antecedents of use richness. Through a field survey with 272 valid responses and structural equation modeling, we empirically tested our model and found that use richness is positively affected by perceived media richness, perceived usefulness, and perceived social usefulness.