Background: Breastfeeding is well known as the optimal source of nutrition for the first year of life. However, suboptimal exclusively breastfeeding rates in the United States are still prevalent. Given the extent of social media use and the accessibility of this type of peer-to-peer support, the role of social networking sites in enabling and supporting breastfeeding mothers needs to be further explored. Objective: This study aimed to leverage mothers' attitudes and behaviors of social media usage to understand effects on breastfeeding outcomes. Methods: Participants were recruited from 1 probreastfeeding social media group with over 6300 members throughout the United States. Online focus group discussions were conducted with 21 women; interviews were conducted with 12 mothers. Qualitative data were aggregated for thematic analysis. Results: Participants indicated that the social media group formed a community of support for breastfeeding, with normalizing breastfeeding, empowerment for breastfeeding, resource for breastfeeding, and shared experiences in breastfeeding as additional themes. Conclusions: According to participants, social media groups can positively influence breastfeeding-related attitudes, knowledge, and behaviors as well as lead to longer duration of breastfeeding. The results of this study should be taken into account when designing interventions for breastfeeding mothers.