1. A method comprising: receiving, by a content publishing system, from each of a plurality of content provider systems, content items for presenting to users; receiving, by the content publishing system, one or more mappings from the content provider systems, each mapping associated with a plurality of users and specifying a weight for each of the plurality of users, wherein the weight is determined by at least one of the plurality of content provider systems, wherein the mapping identifies each user using a content provider specific user id, wherein the content publisher identifies users using content publisher user ids; responsive to a particular user accessing the content provider system via a client device, receiving by the content publishing system, a content provider specific user id for the particular user responsive to the client device receiving a tracking pixel associated with instructions for causing a request to be sent to the content publishing system; for each user identified in the mapping by a content provider system id, identifying by the content publishing system, a content publisher user id corresponding to the content provider user id; storing an association between a content provider specific user id for the particular user and the content publisher user id for the particular user; providing access to content items received from content provider systems to users, comprising, repeatedly: receiving a request from a user via a client device; configuring a web page in response to the received request; identifying a plurality of content items, each content item received from a content provider system; determining one or more weights associated with the user, each weight obtained from a mapping received from the content provider system that provided an identified content item, wherein the content provider system determines the weight based on one or more user interactions performed by the user in association with the content provider system; selecting a content item from the identified plurality of content items based on the determined one or more weights; providing access to the content item via the configured web page; and sending the configured web page to the user via the client device; generating a report aggregating information over users provided access to content items from a particular content provider; and sending the report to the particular content provider.