Social networking site (SNS) inaccessibility remains a barrier for many older adults. Increasingly, research has sought to address these shortcomings with recommendations for design. However, commercial uptake of these findings remains limited, in part, due to the scattering of recommendations across publications, heterogeneity in the SNS systems and features examined, and a lack of sensitivity within the existing guidelines to the heterogeneity of the target demographic. To counter these challenges, we conducted a systematic review following a thematic synthesis approach of 25 empirical studies on SNS design recommendations for older adults. From these, we synthesized a cohesive set of ten distinct design recommendations. These include ensuring an easy-to-use interface, improving social connection features, ensuring personal privacy, and introducing customized features and personalized content. In synthesizing the results, particular care was taken to capture the ways in which population diversity moderates recommendations. The results of this review can serve as a resource for designers and practitioners working on inclusive SNS for older adults. They also highlight the need for additional research into understanding user diversity in relation to SNS accessibility.