This study presents the construction and validation of a formal conceptual model, or domain ontology, useful for the formal representation and analysis of conversations on heritage, memory and identity (HMI) on social network sites (SNS), of interviews with participants in such conversations, and of scholarly works engaging with such phenomena. The ontology provides for the first time a conceptual framework for HM interactions on SNS addressing the semiotic and discursive nature of such interactions in the context of cultural-historical activity theory and semiosphere theory. Part of the Connective Digital Memory in the Borderlands research project, it is developed using an evidence-based knowledge elicitation and domain modeling approach. The study presents the three components of the ontology: an event-centric core conceptual model, an inductively derived concept taxonomy, and a meta-theoretical conceptual scheme, based on a combination of conceptual analysis and lexical analysis of relevant scholarly literature. To validate the ontology, it then provides an example of how it can be used to represent an actual HMI-related SNS conversation and scholarly intervention using knowledge graphs, a quantitative analysis of the occurrence of taxonomy terms in different subfields of HMI on SNS studies, a qualitative analysis of concepts used in studies on non-professional, archeological, and institutional heritage communication on SNS, and a meta-theoretical account of studies of HMI on SNS. The ontology can be used as a framework for theorization and for the development of data models, questionnaire protocols, thematic analysis vocabularies, and analysis queries relevant to HMI on SNS research.