Quantcast claims that SO receives 1.8 mm unique visitors per month, and 17.2 pageviews per month: http://www.quantcast.com/stackoverflow.com
You can't see this part, but Google Analytics shows:
3,768,298 Absolute Unique Visitors 18,180,413 Pageviews
for the last month.
Pageviews seem to match up very closely. Why would there be such a huge difference in unique visitor counts?
(It makes a big deal in the ranking scheme for Quantcast.)