Although Mobile Instant Messaging (MIM) is a massive communication phenomenon and its educational use can be seen as a genuine form of mobile learning, it has been studied to a limited extent to date. The present study examined the use of MIM to engage young professionals in mobile learning communities during their school-to-work transition. This transition is one of the most central but also challenging developmental phases marked by the experience of knowledge gaps and a lack of belonging. To assess knowledge and socio-professional learning effects associated with the use of MIM, this study adopted a quasi-experimental, survey-based approach with an intervention and control condition (n = 114) in the setting of an international research project. In the intervention condition, newly graduated nurses from Nigeria participated in WhatsApp groups in which moderators shared knowledge and stimulated professional discussions over a period of 6 months. Data were collected via online surveys and knowledge tests. The findings show that participants in the moderated WhatsApp groups had significantly higher knowledge and exhibited fewer feelings of professional isolation compared with the control group, which was not subject of any treatment. The effects were even more pronounced when controlling for active contributions (writing vs reading messages), which also amounted to significantly higher levels of professional identification. In addition, across intervention and control groups, the self-reported general active use of WhatsApp (outside of the intervention) was positively associated with the measures of professional social capital maintained with school connections, professional identity, (lower) professional isolation, job satisfaction, and the perceived transfer of school knowledge to work practice. Whereas knowledge and socio-professional effects can be triggered through moderated WhatsApp interventions yet the general (and thus informal) use of WhatsApp is associated with socio-professional connectedness. The findings are of particular relevance in the developing context under investigation, which is marked by a lack of alternative support structures.