Social media platforms are characterized by diverse features and functions, and these facets remain in constant flux over time. This research examines how users define the central purpose of four major platforms in the United States (Facebook, Twitter, Instagram, Snapchat), and how such lay definitions relate to key outcomes previously associated with social media use. In Study 1, we validated self-report measures using a comparative scaling approach to capture what users view as the most defining categories of the four platforms. In Study 2, we investigated whether lay definitions of platforms relate to perceptions of social affordances and social resources. Overall, results provided evidence that defining platforms as social interaction (vs. other categories) is associated with amplified social affordances and resources. Together, the studies contribute to our understanding of how users navigate a dynamic online ecosystem, as well as how lay definitions may anchor the experiences and effects of social media.