The article investigates reasons for the popularity of social networking sites (SNS) among adolescents. International experiences of theoretical understanding and empirical research of motives of SNS usages are reviewed. The paper presents results of a sociological survey among Moscow pupils, conducted by the Laboratory of monitoring studies in 2013. According to findings leading motives of social networks usage are communicative and cognitive ones. It is also shown that recreational opportunities of SNS have a great importance for teenagers, as they form a sense of freedom.