It is commonly said that “there are” social structures on the Internet. But how can they exist there, how can we identify and classify them? A theoretical and methodological framework is presented that describes the relationship between data structures, algorithms, and different types of social structures. We suggest that the latter are “represented” online in different senses of the word: They are not only described, but, by manipulating data, social structures can also be constituted and modified. We then outline a methodology for the analysis of social structures on the Internet: By analyzing the practical meaning of the structures of Internet platforms, we can reconstruct their providers’ and users’ ontological commitments (i.e., what kinds of social structures they have to assume “there are”).