This article presents findings from a qualitative study concerning Australian women's use of Facebook for health and medical information and support and the implications for understanding modes of lay knowledge and expertise. Thinking with feminist new materialism theory, we identify the relational connections, affective forces and agential capacities described by participants as technological affordances came together with human bodily affordances. Affective forces were a dominant feature in users’ accounts. Women were able to make relational connections with peers based on how valid or relevant they found other group members’ expertise and experiences, how supportive other members were, how strong they wanted their personal connection to be and how much privacy they wanted to preserve. We identified three modes of engagement: 1) expertise claims based on appropriation and distribution of biomedical knowledge and experience; 2) sharing experiential knowledge without claiming expertise and 3) evaluation and use of knowledge presented by others principally through observing. We conclude that an ‘expert patient’ is someone who is familiar with the rules of engagement on sites such as Facebook and is able to negotiate and understand the affects and levels of disclosure and intimacy that such engagement demands.