Network contexts have been implicated in the facilitation of syphilis transmission, however little is known about the relationship between online social networks, a proxy for physical networks, and rates of syphilis infection and how they co-evolve over time. To these ends, this study explored the interdependent relationship between Facebook friendship network dynamics and rates of syphilis incidence among young Black men who have sex with men (YBMSM). Specifically, we investigated the extent to which syphilis status, as a latent profile of network members, serves as a selection mechanism of Facebook tie formation and the degree to which Facebook friendships influenced dynamics of syphilis seroconversion. Sexual health, behavioral, and social network data were collected from a cohort of YBMSM (N = 286, Mean age = 22.8) at two time points between 2013 and 2015 in Chicago, IL, USA. The interdependencies between Facebook friendship networks and syphilis infection were assessed using stochastic actor-based models for social networks and behavior. Results showed that YBMSM tended to form Facebook friendships with other YBMSM who had similar syphilis and HIV status profiles, and the hazard of contracting syphilis was likewise influenced by their Facebook friendships, albeit subtly, by being connected to infectious network members. Meanwhile, intrinsic characteristics like condomless sex, HIV status, and online partner-seeking were not associated with dynamics in syphilis incidence. These findings show that there is a mutually dependent relationship between the online network dynamics of YBMSM and rates of syphilis incidence. Findings also direct attention to network-based strategies for intervention.