Among other disclosed subject matter, a computer-implemented method for characterizing user information includes receiving a plurality of identifiers associated with respective users. The method includes identifying, using the plurality of identifiers, any information portions in an information collection relating to at least one of the users, the information collection reflecting network activities by the users. The method includes generating a record that includes the plurality of identifiers associated with the corresponding information portions. The method includes identifying at least one of the information portions as corresponding to a category established for user classification. The method includes identifying a subset of the plurality of identifiers as associated with the category; and. The method includes providing a list to a content provider from whom the plurality of identifiers was received, the list including the subset of the plurality of identifiers and indicating that the subset is associated with the category.