The use of the rel attribute is cursed with the same fundamental problem with meta-data in general.
People can't be bothered to complete meta-data.. and even if they could be bothered, the user generated annotation will be disjointed and rarely follow any standards.
So - I totally agree that with Tim O'Reilly - while this is one step closer, there's a long way to go before its social graph data is as useful as Facebook's social graph.