Social media creates a public space where people with different illnesses can communicate with others experiencing the same conditions and discuss issues that were discussed only in private before. The willingness to share personal health information and private experience is important for successful peer-to-peer health communication but often is prevented by the fear of being stigmatized. The goal of our research was to understand the relationships between the willingness of people with different illnesses to share personal experiences in online communities and the extent of illness stigmatization. We compared online communities devoted to cancer, diabetes, hepatitis B and C, HIV and tuberculosis on the most popular Russian-language social networking site “Vkontakte”. We selected open groups with memberships of more than 1000 users. For selected communities we measured the share of personal stories, the proportion of deactivated or anonymous profiles among users sharing personal experiences and community engagement rates. We found some dependencies between stigmatization of disease and users’ willingness to discuss their personal experiences online. The share of personal stories in the hepatitis and HIV communities was smaller, than in oncology and diabetic communities. But tuberculosis, which occupies a middle position on the scale of stigmatization, showed the largest share of personal stories. The most anonymity was revealed in the communities of highly stigmatized illnesses. We revealed also that a greater willingness of users to share personal experience is associated with a higher engagement rate. Our results contribute to understanding the effects of social media on health.