The ranking is based on visitors, not page views. Based on the graphs posted a few weeks ago at http://imgur.com/a/haHHz, it looks like you have about 2.8 million unique visitors per month to SO's 8.6 million in Quantcast
I checked, the graphs you linked are visits. As of today our visits is at just shy of 11,000,000.
Edit: Ah, you're right. I see, under "visitors". Yeah, that looks about right, we have closer to 3m uniques. So many different terms in stats :D I wonder what the value of uniques vs. page views is.