Social computing provides a variety of challenges and opportunities for people who are experiencing cognitive decline. Following a recent diagnosis of dementia, older adults sometimes engage in online communities designated for people with dementia. In this work, we analyzed all original posts from a seven-year period on a forum for persons with dementia to understand 1) who posts in this forum and 2) what kinds of social support posters seek to gain from this engagement. Our analyses indicate that, in addition to people with dementia, a variety of dementia-related stakeholders create original posts in this forum. Our results suggest that seeking and offering social support are key activities for all forum users, not just those with dementia for whom the forum was designated. Given these findings, we offer design implications for online spaces for vulnerable communities, with a focus on design that allows for external stakeholder participation while still maintaining the privacy and safety of vulnerable members of the community. CCS Concepts: • Human-centered computing → Collaborative and social computing design and evaluation methods;