The Social Internet of Things (S-IoT) is a new paradigm that evolved with a convergence of social networking concepts and internet of things concepts. This paradigm helps develop intelligent devices and services with higher utility. The new paradigm enriches the social interaction between human's objects, heterogenous social objects interaction, the discovery of service and scalability. In recent years, S-IoT has been researching a hot topic and has drawn the attention of researchers, resulting in research on trust management, privacy preservation, service provisioning and security mechanisms. However, S-IoT faces new research challenges like interoperability, trustworthiness, security threats, scalability, navigability, service discovery and composition. In line with the research trend, this article provides a systematic survey of fundamental concepts of S-IoT and IoT, distinguishable characteristics of S-IoT and IoT and research challenges in S-IoT. This survey thoroughly reviews secure communication mechanisms, privacy-preservation techniques and the trust-evaluation model in S-IoT. Furthermore, we accentuate the privacy-preservation mechanism proposed in the literature for seven years and explored research areas. The holistic survey gives an overview of trust-evaluation models with respect to the service provider's perspective. This article overviews secure communication protocols and architecture in the S-IoT context. Finally, various research gaps, applications of S-IoT and research directions help a researcher to carry out research work in S-IoT.