An online system aggregates content item from third party systems for presentation to users of the online system. To do so, the online system accesses the web pages of third party systems to trigger tracking pixels that have been previously placed by the third party systems. A triggered tracking pixel sends a server call to the online system that includes tracking pixel data such as information about the content item that was available on the web page. Based on the information gathered through various tracking pixels, the online system generates a catalog of the content item that can then be presented to a user of the online system such that the user can browse content items of the third party system while accessing the online system.