Background: Although a number of studies have examined social support needs among women with breast cancer, little attention has been paid to the burden and needs for social support among their family caregivers, who often report mental and physical problems associated with caregiving. Objective: This study aims to examine the role of social media in providing social support for offspring caregivers of breast cancer patients. Methods: A peer support group, "Having a breast cancer patient in my family," was created on Douban (www.douban.com), one of China's most popular social media sites, to provide social support to family caregivers of women with breast cancer. We analyzed the content of 784 messages in the discussion threads where the latest update fell between January 2017 and July 2017. Results: The results revealed that the majority of messages (n = 690, 88.0%) provided or requested social support, and more than 64.5% of these messages (n = 445) were posted by caregivers who were offspring of the cancer patients. The results also suggested that these caregivers requested and provided informational support more frequently than they did emotional and instrumental support. Conclusions: This study suggests that social media could be a plausible platform for offspring caregivers of breast cancer patients to share caregiving experiences, access informational resources for their care recipients, gain knowledge about breast cancer prevention, and obtain emotional encouragement. Theoretical as well as practical implications are discussed.