The myriad ways teenagers use social media mystifies anyone who isn’t, well, a teenager. Mary K. Choi wanted to understand the phenomena and make sense of the incrutible, and so she embedded with five high school students. In Like. Flirt. Ghost: A Journey Into the Social Media Lives of Teens, she navigates this rapidly changing, often confusing world. It’s a fascinating look at the rules of social media, from the perspective of those who often write them.