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i had the same experience as yours half year ago. i had banned by G on my main keywords, ie. free widgets, free widget deals, widget deals ...etc. the site is banned on its main keywords because of its exact inbound link title that i built up aggresively through link exchange campaign. before i manage to rank well for those keyphrase, Google already had my site back into sandbox for those highly targeted keyphrase and its ranked way down in the SERP.
does your site's inbound link are mostly the same as well? if so, the penalty is likely the same with mine.
the good news is i did come out from the penalty and recover back to top position through some consistent work and luck. here are the approaches that i had taken in order to hit a 'come-back' after Bourbon:
1. grow my site inbound links *naturally* and varies the inbound link title as much as possible. articles submission, directories ...and so on.
2. link exchange with related sites..and again, VARIES your link title.
3. rebuild the site with a low keypharse density (well, i believe reducing the keypharse density would be enough but what i did half year ago is a 100% reconstruct on my 200pages+ site).
4. add in more contents. the site grew from 200+pages to 300+pages within half year. all pages added are html static webpage with solid content.
that's all what i did and the site came back and rank even better after the Bourbon updates. i think your problem is not that serious as your site still rank well for some of the secondary keywords. there are hopes and all it takes is some patient works and luck i guess.
all the best.
I'd like to mention one thing, that the key word is one word drug name, very competative, it is in top 10 highest competative drugs.
you might be right, I've many affiliate links on the first page, there are about 20 affiliate links on it.
it might be affiliate links filter, like yahoo has.
How many affiliate links do you have? or what is the ratio of affiliate/non-affiliate links?
"Do you think massive link exchange with completely irrelevant anchor text and site deoptimization will help to come out of this kind of ban?"
No, although what i did might makes me looks like i am buying this theory.
what i believe is if a site is too well optimized, Google will send the site back to the sandbox for certain keyphrase. the site is then checked manually or with another set of filter. if the site is found not guilty in the eyes of Google, the sandbox will be lifted.
thus, what i did when i had this problem is to move on and continue growing my site (contents and links building) as well as making sure my SEO strategy (keyword density, h1 h2 h3 tags ..etc) is acceptable by Google.
this, however, is my personal believe based on my own experience. it might not applied the same to other sites nor all of you folks here might agree with.
just 2 cents of mine, hope it helps.
Frankly, I do not buy it. I will believe what I heard right from Cutts' mouth when he said that 'bans' are paranoia and these are just the result of algo shifts. No offense, but I compare these types of threads to the "I was banned from AdSense but i didn't do anything wrong".