This is easily managed by changing the url based on the source of traffic (e.g., the omnipresent ?camId=foo on so many links). Heck, back when I published a magazine, I did this in analog form by using slightly different addresses for mailed in subscriptions (adding a Dept S to the address for the ad in Step by Step, Dept H for How Magazine). It doesn't take intrusive user tracking to be able to measure this sort of thing.