Let's agree on a term:
In a webcomic, each page of comics read is a "pageview".
In a comic book, each page of comics read is a "pageview".
Let's speculate on the following:
The highest-selling American comic books today sell about 100,000 copies. 22 pages x 100,000 = 2,200,000 pageviews per month. Maybe 10 or 15 comics sell in that range.
I could guestimate at least 20 webcomics with more monthly pageviews than that ... and a quick search would find you thousands more with audiences a reasonable fraction of that size.
Has the webcomics readership outgrown the American comic book market?
Someone better at math than I could probably answer this definitively. Croz? Joey? Gary? Jon? Unkie Warren?