Google has reportedly taken action on sites participating in private blog networks, also known as PBNs. On September 18th, Google sent out widespread manual action notices via Google Webmaster Tools to these sites for “thin content” spam.
Private Blog Networks, PBNs, are generally groups of blogs or sites often controlled by one publisher, with the goal of building up links within the network as a way to help particular content rank better in Google. They aren’t really “private” from Google or the public. They can be seen but the “private” name seems to come out of the idea they aren’t really meant for public visitors but instead just to generate links mainly for SEO ranking purposes.
Google considers thin content to be search spam and has a page in their webmaster guidelines explaining they can take action on such type of content.
Some SEOs and Webmasters use these PBNs to artificially inflate their Google rankings either through content spamming or link spam.
It seems as if Google has hit a massive number of web sites utilizing these PBNs over the past several days. Spencer Haws was one webmaster who got hit by the manual action, claiming now he will never use it again.