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#2 in msn and #3 in yahoo.....
1. Check all your outgoing links for deadlinks, changed or damaged sites, others 'gone bad'
2. Check for any 'link exchange' schemes, however innocent they seemed at the time, and remove all traces.
3. Check all reciprocal links; if related and OK, don't worry too much. If directories, lists, 'resources' etc., remove and burn all traces; if other sites, remove.
4. Remove ANY link that you would not genuinely recommend to your visitors.
5. Go back to #1 and check again.
h1 tags...good meta..well thought out linking structure....spider friendly.... etc etc.....
grey hat = link exchanges etc..(areas open to debate on its merits and not defined specifically (enough) in the search engines guidelines.
black hat = cloaking...hidden keywords etc etc...
of course assuming white seo means you shouldn't trip a filter would be wrong...
You didn't give enough information.
All what Quadrille said +
Did the server become inaccessible for a short period of time when Google tried to crawl the site? Did your site change its navigation recently? Did you change anything at all in its directory structure or content layout? Was a valuable link lost? Were the URLs rewritten? Did someone scrape and copy your site? How long since you've been gone, in and out? Did you add anything unrelated to the site recently? Did you add ads? Did you remove ads? Did you add sitewide links to someone? Did someone add sitewide links to you? Did you try to broaden your site's relevancy for things you don't have trusted inbounds for?
... I could come up with 101 other questions but there's just no point. Tell us more.
If you've been scraped a lot, it could also be a duplication issue, and you may not have sufficient link credibility to overcome it. When you disappear, check the search with &filter=0, which may or may not show the problem.
Also, search for dupes of unique text strings in your page copy... not only long strings, but also short ones. Scrapers are breaking up the copy they use.
I recommend that you look up in the Google Hot Topics post, pinned to the top of the Google Search News home page, and read the threads that might apply.
In this case, you might start with this thread, and look at some of the particulars that are discussed....
A Checklist for Sudden Drops in Rank
What do I do next to figure out why my site dropped in ranking?
Canonical issues, dupes, link quality, multiple site issues, server issues, and site changes are, among many others, possible causes.
I suggested to Fugazi that there might be a duplication problem, because scraping is likely if you've been at the top a while for a competitive term. And since he's continuing to rank high in Yahoo and MSN, it might also be a link quality issue. Google tends to be more demanding re inbound link factors than these other two engines.
[edited by: Robert_Charlton at 5:33 am (utc) on April 10, 2007]
It appears to me that Google is looking at inlinks/anchor text and temporarily removing sites it doesn't like. There is much better discussion of this on other threads.
Personally I hope Google gets over this flip-flopping and penalises these sites hard!