Davis (1989) developed a Technology Acceptance Model, a significant construct named "perceived usefulness." It was definedas " The degree to which an individual believes that using a particular system would enhance his or her job performance”, thisconstruct is widely used in the researches about the motivations for using an information technology. In our study, people notonly believes that using social media system would enhance his or her job but also affect their daily life, so we extend "perceivedusefulness" to describe the degree to which people think using social media can be conducive to their communication with othersin daily life and job. If people think social media are conducive to them, namely they think social media is in high perceivedusefulness, they will be glad to use them and get benefits, so it is reasonable to estimate that perceived usefulness can positivelyaffect people’s usage intention of the various functions of social media.