Adults have a hard enough time managing online relationships, said the letter. Younger children, then, are "simply not ready to have social media accounts." Kids, the letter stated, don't really have a full understanding of privacy and what to (and not to) share online with others. It cites studies showing that media use by teens is linked to depression, decreased life satisfaction and body image issues. An app expressly for the under-13 set will likely increase the amount of time younger children spend with digital devices, said the letter, which can end up displacing face-to-face interactions "that are crucial for building healthy developmental skills, including the ability to read human emotion, delay gratification and engage with the physical world."

While the language may seem alarmist, the concerns are likely legitimate. Companies like Apple, Google and Disney have different ways to help parents manage their children's access to screen time already, and even Apple investors are starting to worry about too much time in the digital world. It's not likely Facebook will kill Messenger Kids for anything but a business reason, though the company said it created the app with kid safety in mind. "We worked to create Messenger Kids with an advisory committee of parenting and developmental experts," said Facebook's Antigone Davis in a statement to The Washington Post, "as well as with families themselves and in partnership with National PTA. We continue to be focused on making Messenger Kids the best experience it can be for families."