Digital libraries must support the existing and emergent communities they serve, lest social opportunities to seek, use, and share information and knowledge become diminished compared to physical libraries. Despite many calls for a social view of digital libraries and the rise of social informatics and sociotechnical systems research, there is continuing need to examine how digital libraries support communities and facilitate collaboration. This research improves our understanding of the organizational, cultural, collaborative, and social contexts of digital libraries, conceptualizing social digital libraries to include content, services, and organizations, with a focus on facilitating information and knowledge sharing. A sequential mixed-methods design, drawing from the tenets of social informatics and social constructionism, explores and describes two cases of social digital libraries, LibraryThing and Goodreads, under a theoretical framework focusing on Star's boundary object theory and incorporating Strauss's social worlds perspective and Burnett and Jaeger's theory of information worlds. This framework conceives of social digital libraries adapting to the local needs of many communities, reconciling and translating meanings across them; supporting coherent norms, types, values, behaviors, and organizations; serving as sites and technologies for information behavior and activities; and supporting convergence of broader communities around their use. Content analysis of messages in five LibraryThing and four Goodreads groups, a structured survey of users, and semi-structured qualitative interviews with users identifies three roles LibraryThing and Goodreads play, as boundary objects, in facilitating and supporting translation, coherence, and convergence: (a) establishing community and organizational structure; (b) facilitating users' sharing of information values; and (c) building and maintaining social ties, networks, and community culture. Potential implications for digital library design and practice include highlighting translation processes and resources; providing user profiles and off-topic spaces and encouraging their use; taking a sociotechnical approach that tailors technology and community features to the right audiences; and facilitating the establishment of shared structure, values, and ties and boundary spanning activities. Further research on social digital libraries and in social informatics and information behavior should examine deeper facets of these roles, other digital libraries with less overt social features, and other ICTs in light of the processes of coherence and convergence, taking a boundary-sensitive view of information phenomena in community and collaborative contexts.