In this dissertation the influence of social networking sites (= SNSs) on the daily lives of their users was analyzed. In the theoretical part, the concept of online social capital was developed as a theoretical framework through which to understand and analyze the use of these websites. On the one hand, it was argued that SNSs facilitate the capacity and mobilization of individual information resources. On the other hand, a systematic overview of SNS-associated risks - social pressure, cyber-harassment, deleterious social comparisons, potential for conflict, network demand, data abuse, and problematic usage behaviors - was presented. In the empirical part of the thesis, the size and growth of users` digital facebook networks were analyzed using a facebook application, online surveys and samples of German university students. Results showed linear growth of networks of two contacts per week that was positively related to facebook use. Furthermore, larger networks at the beginning of the semester were positively related to users` number of offline contacts, distance to their last places of residence, Extraversion, identification with fellow students, and facebook use. Users` network sizes and the number of isolated contacts in those networks were positively connected to their perceived online social capital, while the number of isolated groups of contacts (components) and users` betweenness centrality were not. Results also showed that perceived online social capital is an empirically valid concept and positively connected to facebook use, satisfaction with the website, and communication. In general, users indicated little to medium use of facebook for seeking coping-related information and they reported low to medium perceptions of network demand. Theoretical, empirical, and methodological implications of the thesis were discussed.