Abstract: An online system provides content items to content publishers, which present them to individuals who may be online system users. The online system tracks the presentation of each content item and/or actions performed in association with the presentation of each content item using a tracking mechanism and identifies online system users from the individuals based on user-identifying information collected using the tracking mechanism and on information maintained in the online system. The online system determines a number of the online system users having various attributes and extrapolates a set of the individuals associated with each attribute (e.g., based on a sampling method that may de-bias the online system users). The online system generates analytics for the content items and communicates them to online system users associated with the content items, allowing the content items to be tailored to individuals to whom they may be presented by the content publishers.