An online system receives advertisement (“ad”) requests for presentation to its users. An ad request may include one or more values associated with a characteristic of online system users. When an opportunity to present the ad request to a user is identified, the online system obtains the characteristic of the user and determines a value for the user from the ad request based on the obtained characteristic for the user and values associated with the characteristic included in the ad request. The online system generates a score for the ad request based on the determined value for the user, a bid amount in the ad request, and a likelihood of the user interacting with an advertisement in the ad request. Based on the score for the ad request, the online system determines whether to present the advertisement from the ad request to the user.