The metaverse represents a post-reality universe that seamlessly merges physical reality with digital virtuality. It provides a continuous and immersive social networking environment, enabling multi-user engagement and interaction through Extended Reality (XR) technologies, which include Virtual Reality (VR), Augmented Reality (AR), and Mixed Reality (MR). As a novel solution distinct from traditional methods such as mobile-based applications, the technical affordance of XR technologies in shaping multi-user social experiences remains a complex, multifaceted, and multivariate issue that has not yet been thoroughly explored. Additionally, there is a notable absence of mature frameworks and guidelines for designing and developing these multi-user socio-technical systems. Enhancing multi-user social engagement through these technologies remains a significant research challenge. This systematic review aims to address this gap by establishing an analytical framework guided by the PRISMA protocol. It analyzes 88 studies from various disciplines, including computer science, social science, psychology, and the arts, to define the mechanisms and effectiveness of XR technologies in multi-user social engagement. Quantitative methods such as descriptive statistics, correlation statistics, and text mining are used to examine the manifestation of mechanisms, potential system factors, and their effectiveness. Meanwhile, qualitative case studies identify specific measures by which system factors enhance multi-user social engagement. The study provides a pioneering framework for theoretical research and offers practical insights for developing cross-spatiotemporal co-present activities in the metaverse. It also promotes critical reflection on the evolving relationship between humans and this emerging digital universe.
