Attention as Trust
Check out this blog. At a glance, is this spam or not?
It's not until you look down a couple of entries and see the huge list of links that it becomes obvious that it's spam.
Most blog posts today are by spammers..
The only way to tell the real blogs from the spam is by looking at who's reading them. We need a reliable way of verifying that there's a user behind the aggregator; once we know that, we can use their subscription list as a measure of trust that a blog is not spam.