7. The one or more computer-readable media of claim 5 , wherein the instructions, when executed with the one or more processors, further cause the system to at least: maintain metadata and a collection of identifiers of network-based documents comprising the second network-based document, wherein the metadata describes a user characteristic associated with access of users to the second network-based document.
5. One or more computer-readable media comprising instructions that, when executed with one or more processors, cause a system to at least: provide a first network-based document to a computing system associated with a user account, the first network-based document comprising at least an identifier of a second network-based document and code, the code configured at least to, upon execution, determine whether the second network-based document was accessed prior to providing the first network-based document; determine an indication of whether the second network-based document was accessed based at least in part on an execution of the code at the computing system; and associate the user account with a user characteristic based at least in part on the indication.