The forum 4chan receives approximately 9.5 million unique users per month

"Q. And can you describe that briefly, please. What sort of matrix or measurements suggest that it
might be a popular site?
A. As of now in the past 30 days the site received approximately hits from nine and a half million
unique visitors.
Q. And that is during a 30 day cycle?
A. Yes."
Source:  Cross examination of 4chan founder Chris Poole by Mark Krotosk, 22nd April 2010.  Full pdf here
Note - I am assuming that April 2010 was a typical month