Mobile instant messengers, which are types of social networking sites (SNSs), have become popular among smartphone users. This trend prompts Internet-based SNSs to join this wave by launching mobile versions of their applications. The business model of both mobile SNSs is based on advertisements where continuous use of the service represents higher profits. Founded on the theory of uses and gratifications, this research attempts to identify gratifications driving mobile SNSs use. Gratifications were divided into "obtained" - cognitive, hedonic, integrative, social interactive - and "opportunities" - mobile convenience. We collected responses from 330 KakaoTalk and 311 Facebook users. The results shed light on the effect of hedonic, integrative, and mobile convenience in forming attitude. Social interactive gratification exerted a significant effect on all other gratifications-obtained variables whereas mobile convenience had a significant impact on all other gratifications. An exploratory comparison between KakaoTalk and Facebook was performed, and academic and managerial implications were discussed.