Why don't HN and Reddit use PageRank to rank submissions and users?

The PageRank patent has expired, so why not experiment with enhancing voting with PageRank-like algorithms?

For example, when a user upvotes/submits a url, you could have a bidirectional link between the user and the url.

You could then compute the importance of both submitted links and users using a circular notion of importance as is done with PageRank.