Basically, Private Blog Networks are groups of blogs or sites that are immensely used by the other sites for building their links within the network, for ranking their content higher in Google. This is actually not "private" from the view of Google. The word "private" shows that these sites are not meant for public visitors. But in actual these sites are using for generating links for SEO ranking purposes. Many SEOs and Webmasters are using PBNs to increase their Google rankings through link spam or content spamming.
When it comes to the Google, it considers the "thin content" as a "search spam". For this, it has already added a page in their webmaster guidelines explaining the reason behind their action on such type of content. Even, Google has already hit a huge number of sites utilizing these PBNs, taken manual action against them, and claimed penalties against them.
Even though, Google in penalizing the sites, but still some of the PBNs think that quality PBNs get recognization in the Google search ranking. Now, Google has opened up about the PBNs usage and penalties, it is necessary that the sites only use the appropriate and quality link building methods for getting a better search ranking.
To overcome this problem, it is better you take the best SEO Consulting Services. As SEO professionals are experienced in link building and know the Google update effects, they can easily help you to build genuine links for your website.