“It’s happened to me, too,” Lily said, “that guys I don’t even know will approach me online and ask me out. This guy who has maybe a few mutual friends with me will start asking me out and saying, ‘Hi, you’re really pretty, will you go out with me?’ Stuff like that; and it’s really creepy because I don’t know him—it’s like, Why are you talking to me? Ew.” She confessed that another boy had once asked her for nudes. “Just some boy my age, I hardly knew him,” she said. “And I was like, shocked, ’cause like why would you think I would send naked pictures of myself to you when I don’t even know you?

“I didn’t tell anybody except my friends and some of them were, like, ‘Yeah, that’s happened to me, too,’” she said. “I didn’t tell my mom because my parents would freak out and, like, call the police or something. This one friend of mine, she said I should send him a picture of myself nude with my head cut off so nobody would know who I was, but I mean, I would never do that . . .

“I guess it’s my fault for friending him back in the first place,” she went on, “and of course I defriended him immediately after he got creepy. I’ve heard worse things—like this girl in my school, she was on Kik and this older guy who said he was 16 asked her for naked pictures, and he knew she was 14. And, like, how does she even know he’s 16? He could be, like, some 40-year-old man with a big belly.

“I had another situation once,” she said, “there was this really attractive guy who friended me on Facebook and he was really cute, and he said hi to me so I said hi back and we were talking, and he was like, ‘How old are you?’ And I was like, ‘That’s a weird question to ask. How old are you?’ And he was like, ‘Well, I’m 23 and I live in France and I think you’re really pretty.’ And I was like, ‘Nooooo, I’m 14, no.’ There are guys who friend you who are so much older. I mean, that just can’t happen. It’s illegal.

“It’s so easy for older predators to go online and just find a girl; it’s so easy for them to do, because girls want the most friends and they want the most followers and likes, so if someone tries to friend them they’ll just friend them back right away without even knowing who they are. So even if it’s a serial killer, they still friend them back and maybe even start talking to them. It’s scary. Especially since a lot of girls will post pictures of themselves like in their bras and bathing suits, and the people they friend back can see those pictures.

“I don’t do that,” she added. “I know that colleges can see that—anyone can see that; that’s one of the good things about going to a school that educates you about all this stuff, they tell you what everyone can see and what’s appropriate to put online. But some girls don’t know about that and they just put it all out there and they’re not educated enough to know that everyone can see that and anyone can get the wrong idea and just stalk you. Especially with, like, Twitter or Instagram or Facebook, where you, like, tag your location, people can see where you are; and girls tweet out exactly where they are on Twitter, people just see that and they can go and find them.”

Lily’s Facebook and Instagram pictures were mostly selfies; there were many glamour shots of her doing the duckface and “the sparrow,” the name for another selfie expression where the lips are more tightly pursed. There were pictures of her playfully sticking out her tongue to the side, Miley Cyrus–style, and others where she was modestly exposing her cleavage or posing in short shorts. It was as if she were trying to present a sexualized self within the limits of what she knew would be seen as acceptable to parents, colleges.

I asked her why she thought girls posted so many provocative pictures of themselves.

“I think it’s just to get attention,” she said, “to get the likes, everything’s about the likes.”

Adapted from American Girls: Social Media and the Secret Lives of Teenagers, by Nancy Jo Sales, published this month by Alfred A. Knopf, an imprint of the Knopf Doubleday Publishing Group, a division of Penguin Random House LLC; © 2016 by Nancy Jo Sales.