**5. Conclusions**

The implications of this study are both theoretical and practical. Overall, this study makes a unique contribution to the literature on addictive use of social networking sites and broadens the way for further research that may provide additional evidence towards other adolescent-relevant variables. Such research would provide health professionals with relevant information about individual differences in the four types of social networking and internet addiction, and therefore, enriching their professional experience when working with affected adolescents. Furthermore, from a practical point of view, the findings of our study may help psychotherapists to make decisions on who to prioritize a certain intervention and treatment approach based on the relevance of the reported risk factors such as personality and gender. Such stable characteristics may be detected quickly in adolescents in order to identify those who are most at risk and thus start a preventive intervention at an early stage. Thus, the study of youth internet addiction is essential in order to improve prevention and early intervention. Finally, this study has identified important risk factors that underlie the psychological mechanisms of social networking and internet addiction in adolescents. Stressing the benefits of identifying adolescent internet addiction symptoms, problematic social media use, geek behaviour and nomophobia, through a specific instrument that allows for a multidimensional assessment.

**Author Contributions:** All authors contributed equivalently to this research. K.S. and I.M.-C. developed and designed the research, supervising the writing of the manuscript; M.P. and U.d.l.B. were in charge of data collection and conducted the formal analysis; M.P. and U.d.l.B. wrote the first draft of the manuscript; and K.S. and I.M.-C. reviewed, edited, and modified the manuscript critically. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by grants from the Department of Education, Universities and Research of the Basque Government (BFI-2012-40).

**Acknowledgments:** The authors appreciate the participation and support of the secondary schools.

**Conflicts of Interest:** The authors state that there is no conflict of interest.
