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I could understand if my forums were linked to analytics.UGC has run into problems for various reasons. If the sites that are losing ground are forums, it may be part of the reason. I would need to look up specifics but there have been several discussions that involve some perceived or real devaluation for forums.
But the problem is my content has been dropping too. First time 22nd Jan, 2013, recovery in September 2013, second time 19th May, 2014.
Oh my forums have been dying slowly for a couple of years now, which I'm not worried about. I never factor them into my statistics
I have never engaged in link building, and am not sure how or why I would have these links. Our backs are pretty much to the wall over what's happened.
I've read a post on moz.com (I think it's okay to mention that, please remove if it's not), and the author says that when disavowing, you have to own up (I'm paraphrasing) to every link that was made for the purpose of SEO.