We explore how participation in social networking sites (SNS) such as Facebook and Twitter affects the most economically relevant aspect of social capital, trust. First, we empirically test the relationship between SNS use and trust in a large and nationally representative sample. We address endogeneity in the use of SNS by exploiting the variation in the availability of broadband for high-speed Internet, which relates to technological characteristics of the pre-existing voice telecommunication infrastructures. We find that all the proxies of trust significantly decrease with participation in online networks. We discuss several interpretations of the results in light of the specific features of Internet-mediated social interaction and we argue that online incivility may play a major role in the destruction of trust. We then test the role of online incivility in a laboratory experiment. We present the experimental design and some preliminary results.