Today, direct contacts between users are being facilitated by the network-assisted device-to-device (D2D) technology, which employs the omnipresent cellular infrastructure for the purposes of control to facilitate advanced mobile social applications. Together with its undisputed benefits, this novel type of connectivity creates new challenges in constructing meaningful proximity-based services with high levels of user adoption. They call for a comprehensive investigation of user sociality and trust factors jointly with the appropriate technology enablers for secure and trusted D2D communications, especially in the situations where cellular control is not available or reliable at all times. In this article, we study the crucial aspects of social trust associations over proximity-based direct communications technology, with a primary focus on developing a comprehensive proof-of-concept implementation. Our recently-developed prototype delivers rich functionality for dynamic management of security functions in proximate devices, whenever a new device joins a secure group of users or an existing one leaves it. To characterize the behavior of our implemented demonstrator, we evaluate its practical performance in terms of computation and transmission delays from the user perspective. In addition, we outline a research roadmap leveraging our technology-related findings to construct a holistic user perspective behind dynamic, social-aware, and trusted D2D applications and services.