rango - 11:05 am on Jun 8, 2013 (gmt 0)
Just chiming in here since we're in a similar boat. We weren't hit on Nov16th, but since that date there has definitely been a steady downward trend to our traffic to the point where we're at about 40% of the traffic we had this time last year.
Profile: 10+ year old UGC site with forums, photography, user blogs on subdomains, accommodation listings (possible problem area) and a wiki. Search traffic in Nov was at about 500,000 - now at 400,000 per month. Last May the traffic was at 800,000. The drops are hidden a bit by seasonality. I expect this coming Nov to be at about 200,000 assuming nothing else drastic changes.
The recommendation from John Mu to de-index is interesting. We did this during last year on quite a few of our pages and so far there is really no indication that it helped. Quite the contrary - I'd say our traffic has just worsened since then. In other words, tread carefully. I think you want to be very careful which pages you de-index. Blanket rules to deindex all profiles might be dangerous, since you could have some valuable links pointing in there. I don't really think noindex,follow is effective as a way of keeping the link juice flowing. From what I can tell, once Google noindexes it, they rarely look back. I've tried reindexing pages and it's much much harder to do than deindexing them in first place. So maybe first analyze your backlink profile carefully so you don't deindex any of the pages with good links pointing in.