A web of trust would not need to be too deep to start working. After all, it takes just a couple of acquaintance hops to connect 90% of the people on Earth. Not sure about the 6-degrees-of-separation thing, but it's probably on the order of that.
The greater problem of a distributed system would be public key revocation, because you have to go through your entire contacts list to publish a new key.
The point is that you need someone to prevent name conflicts, and that person/organisation is a single point of failure who can be pressured by governments/corporations to revoke names they don't like.
Flagged because the link leads to an error page.
The link is fine, just their server overloaded at the moment.
Yep, looks like Tumblr has some issues at the moment.