1. A method implemented by data processing apparatus, the method comprising: identifying a user device, including determining a device identifier corresponding to the user device; accessing, for the device identifier, visitation data that includes one or more resource locators that each specify a resource that was previously provided to the user device corresponding to the device identifier; identifying, by one or more data processing apparatus and based on one of the resource locators included in the visitation data, a content feed including content items that are eligible to be provided to the user device in response to one or more resource locators in the visitation data matching one or more character strings specified by a publisher associated with the content feed; identifying, from the one or more character strings and by one or more data processing apparatus, a matching character string that matches a particular resource locator from the one or more resource locators included in the visitation data; selecting, by one or more data processing apparatus, a content item from the content items that are eligible to be provided to the user device based on the match between the particular resource locator and the matching character string; and providing, to the user device and by one or more data processing apparatus, data that causes presentation of the selected content item.