Web-based social networking services enable people who share interests to find each other and collaborate in online and offline social activities. Thanks to the widespread popularity of networked handheld devices, constantly carried by members of the public throughout their daily activities, new possibilities of collaboration and interaction are emerging, among people who are not only socially close to each other, but also in physical proximity. However, a challenge arises as to how to enable people with similar interests to find each other, so to fulfill their social activities anywhere and anytime, while constantly moving around. In this paper, we present ADESSO, a semi-distributed directory and matching service that supports opportunistic social networking in delay tolerant networks. ADESSO consists of a set of self-organising brokers, automatically elected based on their mobility patterns. Users offload their requests to perform social activities onto brokers upon encounters; brokers then collaborate, by means of either request exchanges or broker fusion, in order to match activities in a way that satisfies users’ social preferences. Preliminary performance evaluation, conducted using real human mobility traces and social networks, shows that ADESSO generates matches that highly satisfy users’ preferences, entailing only a small overhead.