This is a programmatic paper, marking out two directions in which the study of social media can contribute to broader problems of social science: understanding cultural evolution and understanding collective cognition. Under the first heading, I discuss some difficulties with the usual, adaptationist explanations of cultural phenomena, alternative explanations involving network diffusion effects, and some ways these could be tested using social-media data. Under the second I describe some of the ways in which social media could be used to study how the social organization of an epistemic community supports its collective cognitive performance. Let me begin by considering two1 senses in which we might speak of human thought as being “social”, and how they might orient the study of social information processing and social media. The first sense is a common-place of many schools in the social sciences and humanities: our thought relies on the cultural transmission of cognitive too...