This study attempts to understand information diffusion in online social networks by investigating the factors that affect message dissemination. Particularly, we identify contextual factors from both message and user characteristics on Twitter and examine their impact on diffusion volume and speed. We collected 11,346 tweets, which have been retweeted at least once, regarding the three major mobile carriers in Korea, between September and December 2011. These tweets have generated 59,111 retweets. Our analysis indicates that information diffusion on Twitter in terms of volume and speed is mainly affected by message characteristics such as the inclusion of corporate social responsibility activities. However, the effect of message characteristics on information diffusion is heterogeneous by user type. Our study broadens the knowledge on information diffusion mechanism in online social networks and provides managerial implications on the strategic utilization of online social networks for marketing communications with customers.