Originally Posted by WiredGuy

One possible source of the downward trend I would say is Tumblr. Tumblr used to be a treasure trove of porn and millions of backlinks came from there. Once Tumblr changed their policies towards adult and made adult pages not indexed in Google, links went down with it. This is not likely the only source, but you can bet Tumblr's change in policies had an affect.

WG