Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving a packetized message that is generated by signaling of a tag that is included in code of a given online resource and includes an identifier corresponding to a user that is accessing the given online resource, logging a timestamp as a start of a user session at a website that includes the given online resource, determining that the user session remains active while additional packetized messages continue to be received, determining that the user session at the website has ended, calculating a duration of the user session based on a difference between the timestamp and a time when the user session at the web site was determined to have ended, and modifying distribution of content to the user based on the duration of the user session at the website.