My site has approx 40,000 pages/products. I SEO’d the site and advertised on related content sites (approx. 10) which subsequently gave on average about 10,000 links to about 15 of my important product pages; yes these 10 advertisers have a lot of pages of which my ads appear on most of these pages. This has brought decent traffic to my site These 15 or so product pages began ranking very well in a very competitive industry. These products, keywords, get approximately 90,000 searches per month and average about 1 million results. My 15 product pages would rank anywhere from #15 to #50. Even though I wasn’t getting a lot of conversions, I was definitely getting a lot of traffic. All of a sudden most of these pages are not ranking or rankings have lowered tremendously. I would be surprised if these pages have been penalized because most sites in this industry do this and none seem to have lost any rankings? Any suggestions on the cause would be extremely appreciated.
I take it you have been hit by the Panda update, you must have something going on there Google doesn't like, linking strategy? dupe content? some blackhat? One of my sites was hit the same way, 99.9% of all pages down about 60 pages deep, and this is a completely straight site, it only has 50 odd links so it could just be the end of it's honeymoon period, it's only 6 months old, but with the amount of pages you have I would imagine yours is a lot older, so
Or your competitiors built more links than you = knocked your rankings down, it could be anything, we're not Google. Do you use google webmaster tools?
Check the number of pages indexed in google. Has that gone down? it's probably panda - impossible to tell without looking at the site - but i don't see how changing the linking strategy would have any effect. panda is about site-wide page quality. pages identified as low-quality on large sites, will no longer rank.
I think besides the panda update it could be also the fact that you may not have a privacy policy and terms and conditions.
today one of site hit by these updates too, its was a two year old domain, but i started working on its three months ago, so i think i need to wait for month or two recover these damages.