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; 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 a content provider system that provided an identified content item; and 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.