Google’s John Mueller recently said in a Webmaster Hangout that it’s not enough to create a site and wait for traffic. Similarly, it’s not enough to promote a site then sit back and enjoy the traffic.
Your links and rankings are being subverted by link rot. This isn’t discussed much but it’s something everyone needs to understand and address.
One of the ways to address it is through an approach the late Eric Ward called Link Reclamation. We shared a stage in a presentation about link building and I remember him talking about it.
Link reclamation is the process of “reclaiming” link power that is otherwise going to waste.
A way to do this is to review 404 response codes and check to see if the link is to a web asset that no longer exists.
For example, sometimes sites link to a web page or a PDF file that was moved or no longer exists.
If the web asset has moved, create a 301 redirect to the new destination. Now the link will count as a link to where the new page exists.
If the web asset no longer exists, then how this is treated is a little trickier nowadays.