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, wherein the first UID is associated with a first user platform, and the second UID is associated with a second user platform; 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, comprising to: determine whether a first user associated with the first UID emailed themselves a link to a product to purchase later, wherein the link contains a unique identifier or code specific to the first user, and a second user associated with the second UID accessed the link; and in response to a determination that the first user associated with the first UID emailed themselves the link to the product to purchase later and the second user associated with the second UID accessed the link, 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 a user based on the joined user profile; and a memory coupled to the processor and configured to provide the processor with instructions. a processor configured to: associate a first user identification, UID, and a second UID with one or more Internet Protocol addresses, IPs, wherein the first UID is associated with a first user platform, and the second UID is associated with a second user platform; 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, comprising to: determine whether a first user associated with the first UID emailed themselves a link to a product to purchase later, wherein the link contains a unique identifier or code specific to the first user, and a second user associated with the second UID accessed the link; and in response to a determination that the first user associated with the first UID emailed themselves the link to the product to purchase later and the second user associated with the second UID accessed the link, 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 a user based on the joined user profile; and associate a first user identification, UID, and a second UID with one or more Internet Protocol addresses, IPs, wherein the first UID is associated with a first user platform, and the second UID is associated with a second user platform; 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, comprising to: determine whether a first user associated with the first UID emailed themselves a link to a product to purchase later, wherein the link contains a unique identifier or code specific to the first user, and a second user associated with the second UID accessed the link; and in response to a determination that the first user associated with the first UID emailed themselves the link to the product to purchase later and the second user associated with the second UID accessed the link, join the first and second UIDs; determine whether a first user associated with the first UID emailed themselves a link to a product to purchase later, wherein the link contains a unique identifier or code specific to the first user, and a second user associated with the second UID accessed the link; and in response to a determination that the first user associated with the first UID emailed themselves the link to the product to purchase later and the second user associated with the second UID accessed the link, 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 a user based on the joined user profile; and a memory coupled to the processor and configured to provide the processor with instructions.