Students’ participation in online knowledge construction and interactions with peers has emerged as an important aspect of instructional design. Research in this line confirms that structured interventions can foster this interaction process for high level of knowledge construction to occur. On the other end, in a context where online participation is not obligatory, online participation does not reach high levels of frequency and quality of knowledge sharing. This is usually the observation in adult education (AE) settings wherein an individual trajectory of learning is supported to alleviate workload and time constraints. While there is a lack of studies regarding the dynamics of adult learners’ online participation in a non-obligatory fashion, measurements on online interaction behaviours are mostly qualitative. The present study aims to uncover and contribute to this gap in understanding the motivation behind adult learners’ involvement in online knowledge sharing and their different manifest behaviours in a blended learning context. Thus, their online participation is deconstructed into three types: discussion contribution, discussion facilitation, and social interaction. In addition to sociomotivational variables, we examine online participation from a social capital perspective considering the fact that online participation’s nature in a formal AE context is analogous to that of a virtual learning community (VLC). Participants are 181 learners from different AE centres in Belgium who completed a questionnaire of 54 items developed for the purpose of this study. After the instrument was validated by confirmatory factor analysis (CFA), repeated-measures ANOVAs and multiple regressions were performed. Students exhibited more facilitation behaviours compared to the other two measures of online participation. The regression analysis shows that students’ perceived learning benefits and educational levels are significant predictors of discussion contribution. Employment status has a significant effect on students’ facilitation of discussions, together with perceived learning benefits, whereas altruism has just a marginal effect. As confirmed in most studies, gender and sense of belonging significantly predict students’ social interaction with others. Additionally, students with a higher secondary background were found to be more active than other groups in terms of social interaction. The findings suggest that students’ motivation remains to be the most significant factor and when obligation is not imposed, discussion facilitation is assumed by the students. A social capital framework shows potential to examine online participation, but not to the extent as expected in VLCs. A mutually inclusive learning environment and instructional designs aimed at personally and educationally meaningful activities in combination with certain levels of facilitation and discussion structuring are recommended.