In reality this change happened over one year ago, but only recently it surfaced. Matt Cutts, the head of Google’s Web Spam team, talked about it in the latest SMX event, and soon afterward the whole SEO sphere was commenting. He also wrote a post explaining the whole issue, titled PageRank Sculpting.
Here is a quote from that post that summarizes the change:
So what happens when you have a page with “ten PageRank points” and ten outgoing links, and five of those links are nofollowed? Originally, the five links without nofollow would have flowed two points of PageRank each (in essence, the nofollowed links didn’t count toward the denominator when dividing PageRank by the outdegree of the page). More than a year ago, Google changed how the PageRank flows so that the five links without nofollow would flow one point of PageRank each.
In other words, the usage of the nofollow attribute will still stop the page that is being linked to from receiving PageRank and anchor text value, but the PageRank that is being “saved” with the nofollow links won’t flow to the other links present in the linking page.
If Google wants to discourage PageRank sculpting, I am fine with that. The problem that I see with the change is that it might affect other types of websites negatively, blogs above all.
Because now all the links on your comment section are reducing the PageRank that would otherwise flow to your internal pages.
Here is a numerical example to let you understand it. Suppose that you write a super cool post explaining “Why Obama really won the U.S. presidential election.” The post gets some exposure on social media, and it ends up receiving 200 backlinks from other sites and blogs. Let’s assume that, on average, each backlink was carrying 5 points of PageRank, so the total points of PageRank for that post would be 1,000.
Now let’s suppose that you have 20 navigational links on your layout (e.g., the main menu, the categories and a section with popular posts). This means that each of those internal links would carry 50 points of PageRank to other internal pages on your site, which is good.
The problem comes when people start commenting on the post. Let’s say that, given the popularity of the article, it ends up with 150 comments. Each of those comments carries a link to the site of the comment author. Despite being nofollowed, they would still be counted on the PageRank distribution of that page. Now we have 170 outgoing links, so each link receives only 5.9 points of PageRank. Each of your internal links will only pass 5.9 points of PageRank now, and the total points of PageRank that will remain inside your website are 118.
In other words, before Google changed how it handles the nofollow attribute you would have gained 1,000 points of PageRank with that post. With the change, this number is reduced to 118.
And that is just what would happen in one post. What if we consider a blog with 1,000 posts and tens of thousands of comments? Such a blog could be losing a huge amount of PageRank.
Google might do something to fix this problem, but we still got to hear about it.