Social media platforms are characterized by increasingly diverse features and functions over time. This thesis examines how users define their central qualities – or platform essence – and how those qualities depend on the surrounding media environment. A pilot study and online survey study were conducted via MTurk to validate original measures of platform essence and investigate how the perceived socialness of contemporary platforms shapes key social outcomes tied to popular platforms. Overall, results provide evidence that platform essence – and socialness, in particular – is associated with perceptions of social resources and affordances, bolstering the notion of perceived socialness as a self-fulfilling prophecy. Together, this work makes significant contributions to the existing literature by exploring how individuals navigate their social media ecologies, as well as how lay theories shape the experiences and effects of social media use.