1. A computer-implemented method comprising: receiving, from a user device, a request for content that includes a keyword; identifying, from an inventory of content items, one or more candidate content items that are responsive to the request using the keyword; determining, using one or more processors, for each candidate content item, an expected performance measure associated with a respective candidate content item; comparing, using one or more processors, the expected performance measure for each respective candidate content item to a predetermined threshold; determining that none of the candidate content items have an expected performance measure that exceeds the predetermined threshold; determining that the request for content originated from a user that is not currently logged in to a user account and that a profile of the user is not available; generating, based on the determination that none of the candidate content items have an expected performance measure that exceeds the predetermined threshold, that the request originated from a user that is not currently logged in to a user account, and that the profile of the user is not available, a notification requesting that the user submit information about their interests; and providing, for presentation to the user in a slot of a resource reserved for one of the candidate content items, the notification responsive to the request rather than any of the candidate content items based on none of the candidate content items having an expected performance that exceeds the predetermined threshold, the request having originated from the user that is not currently logged in to the user account, and the profile of the user not being available.