Their core product? The Daily Skimm, a newsletter targeting millennial women that keeps you up to date on what's happening in the world.

They didn't invent the daily newsletter by a long shot, but Weisberg and Zakin are working the kind of magic with it that should make every media executive in the world sit up and take notice. Here's a little context: The New York Times announced recently that it has 13 million subscriptions to its 50 newsletters. Weisberg and Zakin have 5 million subscribers who open their one newsletter multiple times per week.

I caught up with Carly and Danielle at The Skimm's headquarters on 23rd Street in Manhattan, to talk about their journey as business partners and friends, the roots of their entrepreneurial drive, and today's unique challenges for women in the startup game.