Excerpt from this article:
…There is growing evidence that the impact of online social networks is not all good or even benign. A number of studies have begun to find evidence that online networks can have significant detrimental effects. This question is hotly debated, often with conflicting results and usually using limited varieties of subjects, such as undergraduate students.
…“We find that online networking plays a positive role in subjective well-being through its impact on physical interactions, whereas [the use of] social network sites is associated with lower social trust,” they say. “The overall effect of networking on individual welfare is significantly negative,” they conclude.