Online Social Network Services (OSNSs) are changing the fabric of our society, impacting almost every aspect of it. Over the last decades, the aggressive market rivalry has led to the emergence of multiple competing, "closed" OSNSs. As a result, users are trapped in the walled gardens of their OSNS, encountering restrictions about what they can do with their personal data, the people they can interact with and the information they get access to. As an alternative to the platform lock-in, "open" OSNSs promote the adoption of open, standardized APIs. However, users still massively adopt closed OSNSs to benefit from the services' advanced functionalities and/or follow their "friends", although the users' virtual social sphere is ultimately limited by the OSNSs they join. Our work aims at overcoming such a limitation by enabling users to meet and interact beyond the boundary of their OSNSs, including reaching out to "friends" of distinct closed OSNSs. We specifically introduce USNB-Universal Social Network Bus, which revisits the "service bus" paradigm that enables interoperability across computing systems, to address the requirements of "social interoperability". USNB features synthetic profiles and personae for interaction across the boundaries of-closed and open- ,-profile-and non-profile-based-OSNSs through a reference social interaction service. We ran a 1-day workshop with a panel of users who experimented with the USNB prototype to assess the potential benefits of social interoperability for social network users. Results show the positive evaluation of users for USNB, especially as an enabler of applications for civic participation. This further opens up new perspectives for future work, among which, enforcing security and privacy guarantees.