Introduction Informal caregivers of persons with dementia have an increased risk of facing social isolation. Online social media interventions might offer a new opportunity to increase access to social support. An online social support platform, ‘Inlife’, was developed and launched in the Netherlands to enhance social support, positive interactions and information sharing in informal support networks. Objective A process evaluation was performed to evaluate the internal and external validity of the Inlife intervention. Methods Implementation, sampling and intervention quality were evaluated by both qualitative and quantitative methods. Analyses were performed using descriptive statistics and inductive content analysis. Analyses were conducted following participants' completion of the intervention after 16 weeks. Results The overall participation rate in the study was 27% (96/351). The Inlife intervention was generally well-received by the primary caregivers. Inlife facilitated empowerment, openness, involvement, and efficient care organization. Still, adherence was not optimal for all Inlife users. Determinants for Inlife use were identified on the level of the Inlife innovation, the users, and the socio-political context. Conclusions Inlife was evaluated as a useful instrument for efficient central care coordination and mutual involvement. This study emphasizes that the personal attitudes of the Inlife users to seek and provide support warrant attention, next to the characteristics of the actual Inlife innovation for optimal intervention uptake. Online and offline support might be integrated to raise awareness of caregiver social support needs and attitudes and provide insight into caregivers' available social capital. Trial registration Dutch trial register NTR6131, Registered on 20 October 2016.