Andy_Langton - 7:53 pm on Mar 29, 2012 (gmt 0)
I know people claim to do it ... but if it had proven facts a thousand SEO blogs would spout off about the example.
To be blunt, this isn't really an argument at all. If a million SEO blogs said it was true, that wouldn't make it so, and of course, if no-one had ever mentioned it, that wouldn't make it false.
Clearly, no-one is going to post real world data here. But I'm more than happy to echo Martinibuster and say that I also believe that negative SEO - harming competitors is a real world, and effective activity.
One of the reasons that this is an "underground" activity is that, if you ask me, it's legally questionable. The other problem is that, as Martinibuster has suggested, it's difficult to diagnose, and it's an unfortunate aspect of human nature to apportion blame to someone else when things go wrong for you. And it's not that easy. You might just as easily help a competitor as harm them.
But it's really not open to question that links can harm a site's rankings. I think there is enough of a consensus about that (or at least, I really hope there is!). So unless we believe Google has some sort of voodoo where it knows who wrote the cheque to one link network or other, I don't see how there can be any doubt that links can be used to harm a site - whether that's yours or anyone else's.