Matt Buchanan of BuzzFeed wrote:

Last week, tech writer Alexis Madrigal noticed that over half the the traffic coming into The Atlantic was direct traffic to its articles. Since it's unlikely that people would manually type out the URL...

http://www.theatlantic.com/technology/archive/2012/10/dark-social-we-have-the-whole-history-of-the-web-wrong/263523/

...they were obviously coming from somewhere — it's just that no one knows where, precisely. Madrigal proposed that this traffic mostly comes from email and IM, which generally lack referral data, representing "a vast trove of social traffic [that's] essentially invisible to most analytics programs." He grouped all of that direct traffic to article pages together and dubbed it "dark social," in reference to dark matter.

But there's an important distinction: All "dark social" traffic is direct traffic, but not all direct traffic is dark social. In fact the question here is how much of direct traffic really is dark social. Direct traffic, it turns out, is a slippery category: while you might think it's simply traffic from bookmarks and people punching out URLs manually, it's actually any traffic to a page that doesn't tell you where it comes from. There's no referral data, in other words.

tl;dr is that Millennials post links more but Gen X emails links more.

Email will become less significant in the future as posting links becomes higher feedback. Consider the feedback loop: you email something you saw on Facebook, and one person sees it. You post something on Facebook that was emailed, and lot of people see it.

One more thing: Mobile is becoming increasingly important:

The other major finding is that in the BuzzFeed network, nearly 50 percent of direct traffic comes from mobile devices. That's a hugely disproportionate share, since mobile only makes up 23.1 percent of all traffic to the network.

So a lot of the truly "dark social" traffic from email and younger users is in fact happening on mobile devices — which makes a lot of sense, because according to another Pew study, email is still the most popular thing people do with their smartphones.

Also the more social a story is, the less direct traffic it receives:

Across the BuzzFeed network, the top 20 most social stories only have a 45 percent overlap with the top 20 stories with the most direct traffic — and the majority of stories on the top direct traffic list are either celebrity focused (lots of Kate Middleton), among the most searched or long-tail traffic (traffic to older stories). They're less likely to be the kinds of stories that people want to share on social networks with lots of other people, in other words — you might IM your friend a link to Katle Middle n00dz, but you're probably not going to post it on your Facebook wall.

Forgive me, but coming from the adult webmasters POV, mobile is key, the younger demographic is lured a particular way. Instead of thinking of this problem like advertising, I think of it like where do I want to place a virus. I wouldn't want everyone to "see" my message unless they really wanted to see it.  

Well said. Mobile = Hidden in plain sight ...