This article finds that US queer youth of color prefer Tumblr to express intimate feelings and personal politics over other social media such as Facebook. It is based on 5 years of cyberethnographic research in queer Tumblr circulations as well as multi-year rounds of qualitative interviewing with queer youth informants. Several informants experienced drastically negative consequences, to the point of being disowned by their families, because of what this article calls a design bias toward “default publicness” that shapes user experience on social media such as Facebook. This article identifies four design decisions that create “default publicness” on social media platforms, viewing these decisions through queer, feminist, and critical race theories that have argued that the “public” is never neutral terrain. It understands these design decisions as imperatives of “platform capitalism,” which extracts robust and verifiable user data for monetization, and structures these spaces accordingly.