Resumen: his chapter will explore how media companies can use social networks for selling their products and services. We will suggest that the Web 2.0 is an efficient way to create and protect brand recognition, but it cannot be used as a substitutive of traditional marketing channels. First of all, we will analyze the nature of social networks: what are the reasons for their success, what are their competitive advantages and how they built entry barriers. Then, we will look at the expectations of networks' members, considering different typologies: frequent users vs. occasional users, ¿focused¿ networks vs. non specialized networks, young audiences vs. old audiences¿ After that, we will study how some of the most innovative companies are using social networks as a new option of their marketing mix. We will pay attention to cases of success but we also describe some failures and we will try to understand the reasons for those different results. Finally, we will show how media companies can make an efficient use of the most popular social networks to foster the value of their brands and also to avoid a possible spiral of negative opinions from unhappy consumers. We will use descriptive and qualitative methods, including in-depth interviews with managers of companies.