A social networking system determines a predicted supply of ad impressions over a future period of time for social networking system users based on past viewing information of advertisements presented to the users during a prior time period. Based on the predicted supply of ad impressions, the social networking system manages advertisement campaigns including ad requests received from advertisers. The social networking system determines whether the predicted supply of ad impressions is sufficient to satisfy criteria associated with an advertising campaign and provides feedback to an advertiser associated with the advertising campaign indicating whether the predicted supply of ad impressions contains sufficient unallocated expected ad impression opportunities to satisfy criteria associated with the advertising campaign.