This is the third time in recent years that Google has done a major data dump. They have been de-indexing pages for about 2 weeks. Although I only have two precedents since early 2005 to compare this latest data dump to, it may take them up to 2 months to finish the process.

In the meantime, a lot of innocent sites will lose search visibility in Google. People should not panic as this HAS happened before and appears to be following the same pattern.

It's also a possibility that Google is just showing less indexed pages than they actually have indexed, something like the amount of backlinks regarding the link: function. Why else would the traffic remain at the same level?

Google already stopped showing the supplemental pages of websites (ok, could be worked around with a hack), this could be a next step for showing data like this in Webmaster Tools.

"It's also a possibility that Google is just showing less indexed pages than they actually have indexed, something like the amount of backlinks regarding the link: function. Why else would the traffic remain at the same level?"

Since the Supplemental Pages are not allowed to rank for most queries, if the majority of dropped pages are in the Supplemental Index most sites should not see much of a dip in traffic.

However, several of the people in that discussion say they have seen losses in traffic. I've seen some changes in traffic myself, including lost traffic. I've also got sites that have not really lost traffic at all.

"Google already stopped showing the supplemental pages of websites (ok, could be worked around with a hack), this could be a next step for showing data like this in Webmaster Tools."

Google never stopped showing Supplemental pages in site: query results. They just removed the "Supplemental Result" label to make it look like there is only one index.