Using the tracking data of 1,645 smartphone users from Nielsen KoreanClick's panel, this study examines the degree of concentration in smartphone application use in South Korea. The findings from this study are consistent with what we have learned from previous research of usage concentration and selective repertoire in a multichannel and multimedia environment. Overall, the levels of concentration in app usage are high, particularly in the communication and social media categories. Empirical evidence also suggests the existence of network externality in mobile app communication.