With the rapid growth of online social network sites (SNS), the issue of health-related online communities and its social and behavioral outcomes has become increasingly popular in Internet studies and sociology of health and medicine. This paper presents the results of empirical study investigating the structure of the 'friendship' networks and participants' communicative activity within 15 online groups on VK.com SNS. In this pilot study we seek to trace the relation between declared aims (social functions) and structure of online groups devoted to the HIV/AIDS theme. First, we propose a classification of online HIV-related groups according to their declared purposes and actual social functions. The most widespread group types on VK.com SNS are HIV activists, HIV-infected dating groups, AIDS denialists movement groups, online pages of offline organizations and social support groups. Second, we identify and describe several patterns of network structure and user behavior among these groups. We distinguish five types of community structure: tight crowd, polarized crowd, stratified structure, clustered network and disintegrated structure. Finally, we find and interpret relation between the purposes and functions, on the one hand, and network structure of online communities, on the other. Tight crowd networks mainly occur in dating groups for HIV-infected persons, and links in them are determined by users' gender (homogeneous or heterogeneous ties prevail). Stratified structure is related to HIV activists, especially to AIDS denialists movement groups. Crucial factor of network formation for this pattern is participation in public discussion within a group. Active users form a cohesive community while passive users stay isolated or connected with just a few active users. Our findings are consistent with some previous research of communication network structures on other social media platforms.