Nowadays social networking services (SNS) have become an important part of our daily-life. Thanks to the rapid development of mobile computing technologies, more and more people start to use mobile devices, e.g., smartphones and tablets, to access the SNS. Different from the traditional way, i.e., using the desktop PCs to access the web interface of the SNS, using mobile devices will introduce more flexibility. However, there is a lack of a comprehensive study to evaluate the effects of using mobile devices to access SNS. In this paper, we study the Twitter social network by comparing between the emerging mobile users and traditional web users. We have three major findings. First, by examining the tweet posting behavior, we find that mobile users are more active for posting shorter tweets, while less active for retweeting. Second, by referring to the social structure, we can see that the social network composed by mobile users are less clustered, while the radius and diameters of mobile social graph are shorter. Finally, we introduce one classic simulation scenarios, which is information diffusion in SNS. The simulation results indicate that the mobile effects actually decrease the scope of information diffusion as they are less active for message forwarding.