As interest in online learning has increased, studies utilizing a social system for the innovation of lecture/learning environments have attracted attention recently. To establish a sustainable social environment in the online learning system, prior research investigated strategies to improve and manage the social presence of collaborators (e.g., students, AI facilitators, etc.) in an online lecture. Nevertheless, the negative effect of social presence was often neglected, which leads to a lack of comprehensiveness in managing social presence in an online lecturing environment. In the study, we intend to investigate the influence of social presence with both positive (student engagement) and negative (information overload) aspects on the learning experience by formulating a structural equation model. To test the model, we implemented an experimental online lecture system for the introductory session of human–computer interaction, and data from 83 participants were collected. The model was analyzed with Partial Least Square Structural Equation Modeling (PLS-SEM). The result shows the social presence of the collaborators influences both student engagement (other learners: β = 0.239, t = 2.187) and information overload (agent facilitator: β = 0.492, t = 6.163; other learners: β = 0.168, t = 1.672). The result also supports that student engagement is influenced by information overload as well (β = −0.490, t = 3.712). These positive and negative factors of social presence influence learning attainment (student engagement: β = 0.183, t = 1.680), satisfaction (student engagement: β = 0.385, t = 3.649; information overload: β = −0.292, t = 2.343), and learning efficacy (student engagement: β = 0.424, t = 2.543). Thus, it corroborates that a change in the level of social presence influences student engagement and information overload; furthermore, it confirms that the effect of changes in social presence is reflected differently depending on learning attainment and experience.