Building inbound links to your site is a major part of search engine optimization. This is largely due to the weight that Google places on not just the number but the quality of sites linking to your’s. Google’s measure of quality is known as PageRank (PR) and is published infrequently as an integer value between 0 and 10 (though internally at Google it’s calculated continuously and the actual scale is unknown).
When sites link to yours, you inherit a portion of their PR value and, broadly speaking, a single link from a site with a PR of 10 is worth hundreds of links from sites with a PR of 1. Consequently, there’s been considerable effort in the SEO community to find ways to get high PR sites to link back to you. The sites with high PR tend to be:
- Government and education sites (.gov and.edu domains)
- Major social media platforms and content directories (Facebook, Twitter, Wikipedia, etc.)
- News sites
There are many others, and it’s easy to discover a site’s published PR by installing the Google toolbar in Mozilla Firefox – when you visit the site, it’s published PageRank score is shown in the toolbar.
How PageRank is shared
The referring page has a arbitrary and unknown number of PageRank points, which are separate to the PageRank score (the 0 to 10 value mentioned above). As an example, let’s take a page with 100 PR points and 5 outbound links – each link will receive 20% of the flow of PageRank. In the same example, if there were 10 outbound links, each would receive 10%. The key thing to notice here is that the more links on the original page, the less value is transferred to the linked websites.
This means that when you start searching for blogs or forums on high PR websites, with the intention of posting a backlink to your site to receive some of the PR, if there are already large numbers of comments with links, it won’t have a significant impact on the overall PR flowing to your site.
The Importance of Nofollow
Google introduced the nofollow attribute which adds “rel=’nofollow'” to the link. The purpose is to allow sites to restrict who receives a benefit from their PageRank. If you think about this, it’s a clever way to combat content that exists purely to generate a link, such as:
- Adding random comments to blogs to create a backlink.
- Referring to your own articles in Wikipedia’s pages.
- Posting links on Twitter or Facebook’s news feeds.
In fact, Twitter, Facebook and Wikipedia all use nofollow exclusively, so posting links on these sites may have value in terms of generating visitor trafffic, but it absolutely will not help inherit PageRank. Although some in the SEO community dispute whether nofollow has such as draconian effect, Google’s Matt Cutts confirms that nofollow does exactly this (“Nofollow links definitely don’t pass PageRank”).
In terms of how nofollow effects the flow of PageRank, there’s another interesting side effect. In our example above of a site with 100 PR points and 5 outbound links, if 1 is a nofollow and 4 are ‘do-follow’, the do-follow links still only receive 20% of the flow. Why is this? In the calculation, a no-follow link causes PR flow to ‘evaporate’ – and although the amount of evaporation is unknown, it fundamentally causes less PR to flow to the ‘do-follow’ links.
The impact on link building strategy
There are two main lessons here for developing backlinks:
- Creating links from pages with large numbers of outbound links doesn’t offer any material benefit to your own PageRank because of the dilution of PageRank flow, regardless of whether the pages use nofollow in their links.
- Posting links in discussion boards that use nofollow (e.g. Google Groups), news sites (e.g. The Wall St Journal) or any other popular venue that enables you to generate your own no-followed links doesn’t help your PageRank at all.
Given the large numbers of websites, blogs and user-generated content (UGC) sites that implement nofollow now, the days of manipulating content on these to create PageRank are rapidly coming to end. While they may still be useful to encourage traffic, if your goal is to raise your PR, only voluntary links from quality sites that limit their outbound linking will help.