While this is a pretty cool story, I share the author's reservations about what this actually means.

First, Stumbleupon could get more traffic, but less actual interaction. As an example, my personal blog posts recently got about the same number of page views from a post on HN and from a status on Facebook. But, the HN users were on my site for an average of 42 seconds, as opposed to 3.5 minutes for Facebook.

Second, the type of traffic could be different. I have no personal example of this- but it may be that Stumbleupon has more generalist viral content and Facebook could have more tailored content, possibly better for small businesses to monetize.

Either way, really cool for Stumbleupon, I totally did not realize how huge it was.