When even the main domain is on page 6 or whatever, it will drop more. Quantcast requires site owners to implement a tracker, so I guess it's pretty accurate.

Here is some drastic example I found recently (thin/spammy content penalty I guess):

http://extremetracking.com/free?login=fm3u91 http://www.alexa.com/siteinfo/freemp3us.com

From 14,000,000 uniques a month down to ~80 a day :D But obviously Google still has work to do, since that site made a lot of money within that 1-2 months period.

in this case, very accurate. Rapgenius has Quantcast analytics codes on their website.

From my experience. Fairly accurate.