Advertisements are selected for presentation on a search results page based on the presence or absence of user activity within the search results page. When a search results page is provided to an end user, the search results page is tracked for user activity. If user activity is detected, a user context is identified and new advertisements are selected for presentation on the search results page based on the user context. In some embodiments, the search results page provides search results in clusters, and the user context corresponds with a search result cluster selected by an end user. In some embodiments, dominant concepts in the user context are identified and used to select advertisements. Alternatively, if user activity is not detected within a predetermined period of time, new advertisements are selected based on the absence of user activity within the search results page.