Achieving global interoperability among IoT systems has become a very real possibility due to the heterogeneity at all levels of IoT. Besides achieving interoperability, it will become very important to establish social relationships and communications among IoT devices (or things), humans and applications. Social relationships in IoT have been realized through the Social IoT (SIoT) paradigm which is one of the trending feature in the IoT. The SIoT is currently consisted of two types of communications: things-to-things and things-to-human communications; in addition, we propose social cross-domain IoT (SCDIoT), a third type of SIoT communication at a global level which enables application-to-application communication in the IoT. Although interoperability allows the exchange and reuse of data among various applications, it does not focus on the social relationships among IoT applications through which those applications can closely collaborate with each other. SCDIoT fills this gap by operating one level above interoperability. It allows collaboration among IoT applications by enabling them to talk to each other, building social relations and benefitting from each other via various useful services, truly exploiting the advantages of interoperability. We present the concept of SCDIoT, its logical framework and some potential use case scenarios, together with the challenges and possible future research directions.