Editor’s note: Adam Rifkin is co-founder and CEO of PandaWhale, an online network of interesting things and people. Pop quiz: what is the favorite social networking site of Americans under age 25? If you guessed Facebook you are way behind the eight-ball, because Tumblr now enjoys more regular visits from the youth of America. That figure struck me while reading Garry Tan's January 2013 survey and I wondered why? So I delved deeper; this article describes what I discovered while exploring the Tumblr network.