Next, I would check your visitor's behavior. A shift like this flags issues with quality on the site.
- Check to see how long your visitors are on site (if they are reading the content).
- Ensure you have social bookmarking tools to allow your readers to submit to sites like digg, twitter, facebook, stumbleupon, etc... Bookmark bars that stand out to the visitor is highly important.
- Check to see if your content exists on other sites (you can take about 7 words from a article and search google with "your seven words".
- Ensure you have made no changes to your sites navigation from the start of the issue
- If your content is syndicated with other sites, check to see what they are doing and you are not (if your article is ranking higher on sydicated sites and you are the original author of course).
- ensure you are not linking out to any bad neighbor hoods if you allow comments.
- If you are advertising, Google has been known to punish sites for using other advertising sources other than adwords (if linking out). Even on images use rel="nofollow" in your links for those.
I know these suggestions are broad, but do to rules of listing site specifics, I can only guess and hope something said gives insight to your issue.
[edited by: networkliquidators at 10:08 pm (utc) on Nov 8, 2010]