Introduction: Studies conducted over the last decade suggest the importance and use of social media (SM) among individuals with serious mental illnesses (SMI). Individuals with SMI report a desire to use SM to support their mental health; however, little research exists comparing SM use between individuals with and without SMI. Methods: The SMI sample (N=232, average age = 46) consisted of adults with SMI receiving services at 18 outpatient mental health organizations throughout the US. The non-SMI sample (N=300, average age = 52) was recruited from the largest, US-based, private phone/web survey company. Individuals completed a phone survey with questions about SM use. Independent samples t-tests and χ2 tests were conducted to examine group differences. Results: 76.0% of the non-SMI sample reported using SM compared to 33.2% of the SMI sample. Individuals in the non-SMI sample had significantly higher frequency (#times/week), intensity (#hours/day), and longevity (years) of use; and more contacts (e.g., friends/ followers). Facebook was the most-used platform in both samples (92.2% for SMI and 81.4% for non-SMI); and nearly two-thirds of both groups said having a SM account was important to them. Interestingly, 63.5% of the SMI group indicated using SM to feel less lonely compared to only 22.0% of the non-SMI group. Implications: Although individuals with SMI report SM importance, less individuals report a SM account and those with SM use it less frequently than individuals without SMI. The self-reported use of SM to address loneliness may provide suggestions for intervention strategies to increase social connectedness and reduce loneliness among individuals with SMI. Given the potential health-promoting effects of SM, it is important to address SM usage disparities among individuals with SMI. Future research should explore issues related to access as well as issues related to skills and knowledge to use SM technology.