Andylew - 2:21 pm on May 3, 2010 (gmt 0)
New here but felt this important enough to post. I too have noticed a 90% drop in traffic since 1st may, however I belive this is likely to be temporary.
It happened like this:
30th April: webmaster tools started to reduce the 300k pages it was reporting as indexed on the sitemaps by ~50k every couple of hours. So by the end of the day it was reporting no pages listed. Also on 30th april traffic reduced by around 50%.
1st May: Webmaster tools started adding pages ~50k every couple of hours upto a new total of 600k. Traffic reduced by 90%. A site: search still reported the 300k pages.
2nd & 3rd may a Site: search has not changed, an identical number of pages are being returned equal to the number at the end of april. Search terms return the site much further down the listings. Previously the site: search would fluctuate and be identical to the number reported in webmaster tools. Webmaster tools sitemaps fluctuated by ~10k pages which is as expected.
My speculative theory, on 30th april google switched webmaster tools across to caffine system. Webmaster tools now reports pages listed on caffine but the caffine search has not yet been fully activated. Once a site has been transfered that site has a snapshot taken which is used on the old search system, no further updates are done so it slides quickly down the listings.
I belive the reason the clean well seo'd sites are feeling the effects first is that these are the easiest ones for google to digest and update (onto caffine) but doing these first results in reduced positions until the final switch.
A further piece of speculation, the 90% drop only happened on the site that used webmaster tools, I have another which does not use it and has been unaffected.