"I think that social services that are public by default and have huge logged out user bases, should "phantom register" their logged out users by storing activity against their cookies and building user profiles on their logged out users."
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.
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.