Did StumbleUpon just pass Facebook for social media traffic?
thenextweb.com
thenextweb.com
And when someone visits your website from FB, they probably just clicked on a link a friend shared with them (implicitly endorsing you), or came from your FB page, where they have been pre-qualified as interested.
As other commenters have pointed out, pageviews aren't the best way to measure the efficacy of your digital promotions. Try your best to find metrics that align with your actual business goals. Conversions to sales would be a good one, but something like time spent on site or pages viewed per session are fair indicators that you're doing something right.
I'm not trying to decry SU as worthless or this article as baseless; not by any means. I just want to encourage a healthy skepticism of metrics that don't necessarily mean anything. If "that which gets measured, gets managed", then optimizing for page views could mean you find yourself in the deadpool soon.
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.
Someone who is reading a link that a friend has sent them is expected to read it, otherwise the friend may be unimpressed or you might not have anything to talk about. Plus, there is the added incentive of impressing or gaining a connect to the friend or appearing interested in your crush/gf's interests.
On HN and reddit, my only interest is whether I want to learn something or make myself laugh.