Link ranks are our way of measuring the strength, persistence, and vitality of links appearing in weblogs. When PubSub reads a new weblog entry, we pull out any URIs we find and attach them to the entry in a separate field. This allows our users to include domain names or linked file types when creating subscriptions.
From this set of URIs, it's easy to find the most popular domains. Link ranks take one more step and calculate scores for each linking site; domains are then scored based on the values of the sites that link to them. The theory is basically that these are the links you're most likely to click on, if you read a weblog at random.
Unlike Google's PageRank system, link ranks are not iterative. Rather, we base link ranks on a simple formula that only looks at local links - links which are within one or two steps of any target site. Also, it's important to note that we only look at links which are in weblog entries - we don't read any of the other links on the page, like the side bars or blogrolls.
The intent of this system is not to measure the strength of any particular domain, but rather the relative likelihood that you'd find and follow a link to that domain. As such, the links are what's really important, not the pages themselves.
To calculate link ranks, we generate a link score for each domain. Link scores are calculated in three steps: first, we find a point value for every site that links to other sites. Second, we use the point values to generate link scores for each domain. Finally, we weight the daily scores over a fixed period to arrive at an aggregate score for the site - this ensures that more recent links are given more value than links from several days ago.