This article examines epistemic impacts of social media, merging Gibson’s affordance theory with the notion of selective permeability, which holds people encounter objective differences in a setting because of their distinct capacities, only here applying the idea to online spaces. I start by circumscribing my deployment of “affordances,” taking care not to totally divorce the term from Gibson’s intent, as often happens in information technologies research. I next detail ways that selective permeability characterizes online epistemic landscapes, focusing on how factors (...) (like culture) moderate normative standards for sharing information. This leads to a discussion about selective receptivity and blindness to information in the context of social media—a situation that amplifies political divides and renders antagonists mutually incomprehensible. This segues into my final topic: an exposition of how social media can make the offline world more selectively hostile to some. An additional proposition is that practical habits cultivated on networking apps can modulate what affordances are present in offline arenas, as opposed to just affecting which ones get noticed. Throughout, I suggest that selective permeability answers an increasingly recognized challenge: the difficulty of generalizing conclusions about one app to others and across cultures, ages, etc. Ultimately, my aim is not only to explicate how social media fragments common social knowledge but to defend selective permeability as an epistemic template for comprehending social media. (shrink)