An improved user modeling server for use in media delivery networks, wherein a plurality of applications exchange usage data via at least one user modeling server. The user modeling server has an associated function for regulating exchange of usage data between any of a first application and a second application in the set, wherein the function defines: whether the usage data are provided by the first application to the second application, and a degree of trust acknowledged by the second application to the provided usage data. The function is in the form of a bi-directional relationship, wherein any of the first and second applications is configured for accepting, refusing or negotiating the relationship. The function is preferably configured for valuating the usage data, and defining debit and credit values each application has in respect to usage data exchanged with other applications in the set.