Today, the rapid adoption of mobile social networking is changing how and where humans communicate. As a result, over the recent years we have been increasingly moving from physical (e.g., face-to-face) to virtual interaction. However, there is also a new emerging category of social applications that take advantage of both worlds, i.e. using virtual interaction to enhance physical interaction. This novel form of networking is enabled by device-to-device (D2D) communication between/among the laptops, smartphones, and wearables of persons in proximity of each other. Unfortunately, it remained limited by the fact that most people are simply not aware of the many potential virtual opportunities in their proximity at any given time. This is a result of the very real digital privacy and security concerns surrounding direct communication between ”stranger” devices. Fortunately, these concerns can be mitigated with the help of a centralized, trusted entity, such as a cellular service provider, which can not only authenticate and protect the privacy of devices involved into D2D communication, but also facilitate the discovery of device capabilities and their available content. This article offers an extensive research summary behind this type of ”cellular-assisted” D2D communication, detailing the enabling technology and its implementation, relevant usage scenarios, security challenges, and user experience observations from large-scale deployments.