1. A system for cross platform user joining, comprising: a processor configured to: associate a first user identification (UID) and a second UID with one or more Internet Protocol addresses (IPs); associate the first UID and the second UID with one or more monitored behaviors; join the first UID and the second UID based on the one or more IPs and the one or more monitored behaviors, wherein the joining of the first UID and the second UID comprises to: determine whether a first user platform associated with the first UID emailed a user an email including a link including a product to purchase later and a second user platform associated with the second UID opened, from the email, the link including the product to purchase later, the link including a unique identifier or a code specific to the user; and in the event that the first user platform associated with the first UID emailed the user the email including the link including the product to purchase later and the second user platform associated with the second UID opened, from the email, the link including the product to purchase later, join the first and second UIDs; generate a joined user profile based on the first UID and the second UID; and send the joined user profile to a web service, wherein the web service can customize content presented by the web service to the user based on the joined user profile; and a memory coupled to the processor and configured to provide the processor with instructions.