As we wait with anticipation for the elevator doors to open at the New York City headquarters of theSkimm, we're immediately greeted by heart-pumping music. The playlist: songs that were popular in 2012, the year that Carly Zakin and Danielle Weisberg sent their first email newsletter as theSkimm. These days, theSkimm has more than five million subscribers to its daily sum-up of the news.