The stories that fall between the cracks are those that are passed around in casual ways, outside the social media super-structure, millions of times a day. "Most of the time," writes Alexis Madrigal, "someone Gchatted someone a link, or it came in on a big email distribution list, or your dad sent it to you."

Alexis, who is the tech editor at TheAtlantic.com, gave a name to this black hole of traffic: dark social. These sorts of referrals are not broken out in the chart above. But they represent a large part of the audiences we all receive. At The Atlantic, our director of analytics, Adam Felder, estimates that dark social accounts for about one-quarter of all referrals to our sites.

I wouldn't call it a "black hole of traffic"!

It's a lot of traffic and even though we don't have websites as referrals, email referrals count for a lot!

It is an excellent expression of the Interest Graph when someone is willing to email a link to another person.