fred9989 - 10:29 am on Apr 5, 2010 (gmt 0)
Could it be the drop in traffic is partly due to reduction in long tail searches?
The size of the supplementary index seems to have been vastly increased (c. doubled in fact), which implies many pages in the regular index are now not showing up as they would have been quite recently.
For example, on one Google search in my sector, the message "In order to show you the most relevant results, we have omitted some entries very similar to the nnn already displayed.
If you like, you can repeat the search with the omitted results included." now appears after 400 or so searches rather than 900.