Health care in aging societies increasingly demands that relatives, partners, or friends provide informal care for loved ones at their end of life. Yet, being an informal caregiver involves significant health threats caused by so-called caregiver burden. To cope with the broad spectrum of challenges, informal caregivers seek social support in the care relationship network emerging around a (future) patient. However, obtaining social support is not limited to offline contexts. Members of online communities also provide experiential knowledge and social support. To explore how informal caregivers seek and provide social support online and how this is interrelated with their care relationship networks, we conducted a qualitative content analysis of 75 threads about advance care planning from German online forums (2003-2017). Our findings show that informal caregivers rely on what we conclusively coined communicative care (i.e., informational and emotional support in burdensome care situations), often in response to impaired offline relationships within care relationship networks.