In general, a hidden backlink is a backlink which is invisible to third party crawlers of backlink checker services like ahrefs, Majestic, Moz, SEMrush, Sistrix and co but not to Googlebot. A hidden backlink passes link juice like any other backlink because Google does not notice any difference. We just named them hidden backlinks because such backlinks are hidden from backlink checkers SEO experts use. Hidden backlinks are not hidden from Google.
Because a hidden backlink passes link juice like any other backlink. There is no difference from a search engine perspective. If you just monitor visible backlinks, you are most likely missing a part. How big the influence of hidden backlinks are, depends heavily on each website. Even if a website does not have hidden backlinks pointing to it, it's good to know.
The Linxact crawler detects hiding methods facilitating robots.txt disallow entries, meta noindex flags, .htaccess rules and even server-side code that hide outbound links from common backlink checkers.
The precision of our data is determined by our crawl scope, crawl depth, filtering scope and hiding method detection rate.
Crawl scope: Our current crawl is scope is ~500 million domains. This is already pretty impressive, but of course we will improve this in the future.
Crawl depth: At this moment, we crawl root domains 3 hops and subdomains 1 hop deep. In the context of hidden backlinks, subdomains are not that important compared to root domains. Soon we will crawl root domains 4 hops deep, which will result in much more crawled hidden backlinks. Stay tuned!
Filtering scope: To reduce costs, we do not crawl Asian websites. We filter them out with content and TLD filters. We are also excluding spam websites from being crawled deeper than 1 hop by using Machine Learning classification models.
Hiding method detection rate: Our detection rate is pretty good, but not perfect yet. There are a few edge cases where it might be possible to bypass our detection algorithm, but we did not encounter those methods in the wild.
We do not offer our own off-page metric, and we do not plan to in the near future. An off-page metric is useless without common backlink data because common backlinks make up a big chunk of all backlinks. Linxact allows you to export hidden backlinks as CSV file which could in turn be imported to a common backlink checker like ahrefs, Majestic, Moz or SEMrush in order to evaluate their SEO strength. In addition, we plan to integrate a third party off-page metric.
A Private Blog Network (PBN) is a very popular Black Hat SEO method. A PBN is a set of websites controlled by the same entity, which uses this network of websites to link out to a profit generating website. With that method, a Black Hat SEO can rank his profit generating website without organic links. Often a PBN website is built upon an expired domain which already holds some SEO strength.
There will always be a percentage of websites blocking third party crawlers to reduce server load and other reasons.
Hiding outbound links on PBN websites will not go away either because only a limited number of SEO professionals will use our service. There are no signs of a decrease in hidden backlinks. In fact, we encounter a steady and continuous growth in hidden backlink building.