This paper deals with the sign up issue in social networks populated with Web services. These social networks can be used for example, to ease the discovery of Web services. Based on Web services’ functionalities three social networks are built: competition, substitution, and collaboration. In competition and substitution social networks, Web services offer homogeneous functionalities. In the collaboration social network, Web services that offer heterogeneous functionalities. In this latter type, Web services can be put together to develop composite services. Prior to joining a social network, a Web service through a third-party, named social Web service, should evaluate the pros and cons of being member in this network. A set of quality criteria for assessing these pros and cons are proposed. These criteria include, but are not limited to, privacy, trust, fairness, and traceability. Policies for managing the sign up are, also, provided in this paper. The adoption and efficiency of these policies are monitored and assessed with respect to the values that these criteria take. In response to this sign up’s outcomes, these policies are fine-tuned.