Unique user impressions of advertisements on a social networking system may be tracked and approximated by the social networking system. After an advertisement is delivered to a user, identifying information about the advertisement is associated with the user if the advertisement was not previously presented to the user. An impression count records the number of unique impressions of the advertisement by users of the social networking system up to a predetermined threshold. After the predetermined threshold is reached, the social networking system records a sampling of the unique impressions. Upon request for the number of unique impressions for an advertisement, the social networking system may extrapolate the recorded unique impressions based on the predetermined threshold. In one embodiment, a time degradation factor may be applied to the sampling rate so that recent impressions are sampled more than older impressions.