What about if the user isn't allowing data to be stored, is using a vpn or proxy, a dynamic IP, or something else that prevents you from "storing activity"/comparing/etc.. I've seen this done before to target advertising to phantom users on adult sites, it doesn't work. Most of those people who aren't logging don't won't to log in/participate and "comparing activity" isn't exactly a piece of cake and is depending on those users having cooperating connections. You might argue that these people are fringe users but even then I doubt the ability/feasibility to accurately retain and compare data usefully and not just using IP or something to compare visits.