Social Internetworking Systems are a significantly emerging new reality; they group together some social networks and allow their users to share resources, to acquire opinions and, more in general, to interact, even if these users belong to different social networks and, therefore, did not previously know each other. In this context, owing to the huge dimension of existing social networks, the capability of a Social Internetworking System to provide its users with recommendations of reliable users and social networks, as well as of high-quality resources, is extremely relevant. In the past, user and resource recommendation has been investigated in the context of a single social network, whereas it has still received a little attention in the context of a Social Internetworking System, owing to the novelty of this phenomenon. For the same reason, social network recommendation has received an even less attention. In this paper we propose a trust-based approach to face these challenges. Specifically, we introduce a model to represent and handle trust and reputation in a Social Internetworking System and propose an approach that exploits these parameters to compute the reliability of a user or a social network, as well as the quality of a resource. These last measures are then exploited to perform recommendations.