We present findings from a qualitative study of activity on Pinterest.com, in which we investigated professional and personal uses of the site using interview data and observations of online activity. We find that Pinterest serves as an infrastructure for repository building that supports a wide range of activities including: discovery, collecting, collaborating, and publishing. We discuss these concepts using the language of “boundary objects” from the sociology of science. We suggest that scale is a critical dimension of boundary objects for understanding how people make sense of Pinterest and their diverse goals for using it. Professionals often attempt to use Pinterest to create repositories that scale to groups, organizations and societies and interface with multiple social worlds whereas personal repositories often have highly localized meanings. Our approach builds on quantitative descriptions of Pinterest to understand how the site fits into a growing ecology of social network sites.