Socialization plays an essential role in ensuring that newcomers to an organization enjoy greater productivity and also integrate well into their workgroups. Since the COVID-19 pandemic, enterprise social media (ESM) is increasingly being used in organizations as an informal socialization tool. However, the effectiveness of these tools in affording the necessary socialization opportunities and enabling the newcomers to better integrate within their organizational settings remains both unclear and understudied. Drawing on the extant literature on newcomer socialization within organizational behavior and the IT affordances research, this study proposes a multilevel research model to explain how various ESM affordances affect newcomers’ socialization effectiveness (i.e., performance proficiency at the individual level and group cohesion at the group level). Plans for verifying the model using two quantitative sub-studies based on the multi-source and multi-wave research design are also presented. Research implications and future research plans are also discussed.