1. A method, comprising: identifying, from a first source, advertisement log data that includes a first timestamp specifying when an advertisement was provided to a user device having a given Internet Protocol (IP) address, wherein the advertisement log data does not include a user ID of a user of the user device; identifying, based on browsing log data from a second source, an online resource that was accessed by the user device having the given IP address and a second timestamp specifying when the online resource was accessed by the user device having the given IP address and the user ID of the user of the user device, wherein the browsing log data does not include information specifying the advertisement that was provided to the user device having the given IP address; determining, based on the second timestamp specifying when the online resource was accessed by the user device and the first timestamp specifying when the advertisement was provided to the user device having the given IP address, that a time difference between the first timestamp and the second timestamp is less than a specified time difference, wherein the second timestamp specifies a different time than the first timestamp; in response to determining that the time difference between the first timestamp and the second timestamp is less than the specified time difference, merging the advertisement log data and the browsing data to create a merged data log that matches a given user ID from the browsing log data with a presentation of the advertisement at the user device based on the given IP address being included in both of the advertisement log data specifying that the advertisement was provided to the user device and the browsing log data specifying that the online resource was provided to the user device; and outputting, over a communications network and to a client device, data that presents, at the client device, one or more content items based on the merged log data that includes both of the user ID and the information specifying that the advertisement was provided to the user device having the given user ID.