Recently there has been a dramatic proliferation on the usage of social networking websites, blogs and other information systems that have a social orientation. However, there is limited research on what drives individual to use such information systems and moreover what are the outcomes of their usage. This paper develops and empirically tests a model of user acceptance of social information systems. Taking into account the particular characteristics of these systems, the proposed model has several features that extend technology acceptance literature. Firstly, instead of the behavioral intention as the key dependent variable, the rate and the variety of use are used in order to model technology acceptance based on the use-diffusion theory. Secondly, exploiting a relational perspective of user-technology interaction the concept of user stickiness with an information system is investigated as usage outcome. Finally, the concept of addiction tendency is proposed to be another outcome modeling the excessive and intensive use of social information systems. In order to empirically test the proposed model a survey with the participation of 456 facebook users is conducted. Results, analyzed with structural equation modeling, confirm study’s hypotheses and show that social information systems could be a new field for theorizing about human behavior and technology.