An advertising system identifies users associated with an interest topic and generates a list of such users in which all advertising accounts are proportionately represented in the list. Such users are identified by recording user-page access data to each page in a cluster of pages associated with the interest topic. A list of user-account associations is generated by grouping the user-page access data by the advertising account associated with each page. The list is then optimized so a proportion of user-account associations for each advertising account is less than or equal to a predetermined threshold. This ensures that no one advertising account is overrepresented in the list. Using the optimized list, the advertising system can target users associated with the list with advertisements related to the interest topic.