1. A computer-implemented method of determining the similarity between entities across different identifier spaces, the method comprising: building a first model specific to a first identifier space based at least on a first set of features associated with an archetypical population operating in the first identifier space, the first set of features determined based on histories associated with the archetypical population in the first identifier space; identifying a join panel of entities that each operates in both the first identifier space and a second identifier space, each entity of the join panel having a respective first identifier associated with the first identifier space mapped to a respective second identifier associated with the second identifier space; computing a score for each entity of the join panel by applying the first model to each entity of the join panel, each respective score reflective of the similarity between the respective entity of the join panel and the archetypical population; identifying a second set of features associated with the entities of the join panel based on the scores, the second set of features determined based on histories associated with the entities of the join panel in the second identifier space; building a second model specific to the second identifier space based at least on the second set of features; applying the second model to a target entity operating in the second identifier space to compute a score for the target entity, the score indicating the similarity between the target entity operating in the second identifier space and the archetypical population operating in the first identifier space, wherein an identifier associated with the target entity in the second identifier space is not mapped to an identifier in the first identifier space; responsive to the score for the target entity indicating the target entity is similar to the archetypical population, targeting the target entity to receive advertising content; and sending the advertising content to the target entity.