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Two large forum sites (not exclusively forums but the number of forum URLs is much larger than the rest of the site). Started dropping on the 16th, on the 17th of November 2012 traffic stabilized at 40% loss on one and 50% loss on the other.
My initial impression was also low quality UGC - I sort of let moderation slip during the election time and a lot of political spam (completely OT and some of it quite outrageous) got in anywhere from early summer all the way up to early Nov. Deleted what I could find but no movement as yet.
Traffic appears to be experiencing what others describe as the glass ceiling effect. A THICK glass ceiling if I may add. A day could start fine, almost like before Nov 17, but it will come to just a trickle around 5-6PM EST time. If it starts mediocre, it would stay that way whole day. Anyhow, the end result in both cases - 50% of the pre-Nov 17th traffic.
I am honestly at a loss at what could possibly be done regarding low quality content on a forum site: it's not like you can post a sticky saying:"dudes, you've gotta put more thought into your posts!". Lots of posts are (and have always been) very short, some silly, some not-so-obvious spam, some people just post "+1" or "what he said" - nothing I can do about it whatsoever.
Interestingly, when I check some of the top keywords, the SERPS appear to be the same, as well as the average position reading in WMT - but still 50% less traffic coming through on them. What could be causing this? Do they now prefer not to show a site where previously they would lower the SERP instead? Can't find another explanation of the unchanged "average position".