We developed user models of knowledge exploration in a social tagging system to test the expertise rankings generated by a link-structure method and a semantic-structure method. The link-structure method assumed a referential definition of expertise, in which experts were users who tagged resources that were frequently tagged by other experts; the semantic-structure method assumed a representational definition of expertise, in which experts were users who had better knowledge of a particular domain and were better at assigning distinctive tags associated with certain domain-specific resources. Simulations results showed that the two methods of expert identification, although based on different assumptions, were in general consistent but did show significant differences. As expected, the link-structure method was better at facilitating exploration of popular "hot" topics than the semanticstructure method. However, the semantic-structure method was better at guiding users to fi...