Techniques and technologies for mapping user identifiers between different device ecosystems are described. In at least some embodiments, a system includes a processing component, a memory, and a mapping engine configured to map a first identifier associated with a user with a second identifier, at least one of the first or second identifiers being associated with a first device ecosystem; and map a third identifier associated with a second device ecosystem with at least one of the first identifier or the second identifier, the second device ecosystem different from the first device ecosystem, based at least partially on (i) a plurality of first co-location occurrences of the third identifier with the first identifier, and (ii) a plurality of second co-location occurrences of the third identifier with the second identifier. The mapping engine may provide an indication of an association of the third identifier with the user.