Gary Illyes and John Mueller from Google both mock a tweet about PBNs, private blog networks. The tweet contains a screen shot of a private Facebook message saying that he was able to block the PBNs by using robots.txt and "it worked." It suggests that whatever issue the webmaster had, after they blocked the links and content from the PBNs, their rankings got better.
Here is the tweet with the screen grab from Facebook:
Gary Illyes from Google retweeted it and said "true story."
True story! https://t.co/g1p0ufM1rU— Gary Illyes ᕕ( ᐛ )ᕗ (@methode) March 12, 2017
John Mueller from Google replied also:
Internally at black hat forums, there are always debates if PBNs are still working or not. Overall, most of those threads win out as if they do work. I do not know but clearly PBNs have been and still are on Google's radar - which can be a scary thing for those playing that game.
Forum discussion at Twitter.