> We will also never share the content of your historified pages with anyone, although we might use the URLs of pages to compile anonymized lists of popular or new links. We will, however, never share any information about which pages you, personally, have historified.
I think that might be a mistake. You'll lose the ability to get people in for social networking/SEO reasons, and nobody trusts a service like this to stay private any more anyways. At first I thought, "Well, that's more useful because it's private" - and then I thought, "Who am I kidding? They'll just make it public anyways if they get big."
You might consider setting the default to public, maybe with an option for private.