Social media develops a common asymmetry: few contribute much, the majority little. By "participatory asymmetry" we understand this long tail of participatory production as an interactive dynamic among two types of social media users, regular and sporadic, that shapes the curation of social media spaces. Examining online discussions surrounding massive, open, online courses (MOOCs), we observe interactive differences between regular and sporadic users that shape patterns of social media curation across the networked discussion forums of a major, online newspaper. Whereas regular users reply directly to other users to durably articulate discussions as "media objects;" sporadic users post one-to-many messages which aggregate in platform-reconfigurable cascades of social media or "media flows." By mapping the relationship between regular and sporadic participation, one-to-one and one-to-many interaction, and the curation of social media spaces in patterns of media objects and flows, we propose a four-part typology of participatory asymmetry: broadcast, feedback, moderated, and dialogic.