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. This does two things. First, if those logged out users eventually register and become logged in users, this "phantom profile" can help the user get a lot of value from the service right away. And second, this "phantom registration" might allow the service to permit lightweight engagement without logging in. Lightweight engagement might be favoriting an item on Etsy, hearting something on Tumblr, or starring something on Twitter.
Great point. Of course, this may well be technically illegal in the EU.