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Are kids ready for social media?

Until December, the answer was no. Even Facebook had rules against children under the age of 13 joining more than 2 billion other people around the world who logged on to the service each month. But since then, the company has created an app called "Messenger Kids," designed to let children communicate with people approved by their parents.

Now a group of child advocates is sounding an alarm about Messenger Kids, saying young people should be shielded from these types of services until they're older.

"Younger children are simply not ready to have social media accounts," Campaign for a Commercial-Free Childhood, a Boston-based nonprofit advocacy group, said Tuesday in a letter.

The letter was signed by 19 other organizations, including the ACLU, Parents Across America and the Badass Teachers Association, as well as dozens of individual experts on children. It was addressed to Facebook CEO Mark Zuckerberg, himself the parent of two children under the age of five.

The move marks the latest in a long-running debate about how quickly children are exposed to technology and internet culture. Childhood development experts have debated for years about how much time, for example, children should be allowed to watch TV or interact with computers and tablets.

Technology addiction is also a concern. Earlier this month, Apple investors told CEO Tim Cook that he hasn't done enough to protect young people from getting addicted to phones, and they asked the iPhone maker to study the device's effects.

Part of the problem is the lack of definitive answers and sufficient research. The iPhone, after all, is only 10 years old.

Last year, the American Academy of Pediatrics revised its rules for technology use by kids, relaxing some of its recommendations to account for the explosion of apps and interactive programs meant for children.

Facebook isn't the only online service that's become a concern to child advocates. Google's YouTube Kids app, ostensibly designed to be a safer kid-focused version of YouTube, actually allowed inappropriate items to slip through.

In terms of social media, there are also questions about when and how children should be exposed to sites like Facebook, with its addicting likes and shares from friends. That brings us back to Facebook and its Messenger Kids effort.

Facebook says it designed Messenger Kids to help parents and children chat in a safe way and give parents control of their kids' contacts and interactions.

"We worked to create Messenger Kids with an advisory committee of parenting and developmental experts, as well as with families themselves and in partnership with the PTA," a Facebook spokesman said in a statement, referencing the National Parent Teacher Association. "We continue to be focused on making Messenger Kids be the best experience it can be for families. We have been very clear that there is no advertising in Messenger Kids."

Not everyone is convinced. Groups like Common Sense Media, which rates and researches various shows, movies, games and other media, focused its concerns on the way apps like Facebook are designed, and how they influence Messenger Kids.

"We know that social media companies use manipulative tactics to keep all of us coming back for more, in spite of the mounting research that points to negative consequences," said Common Sense Media's CEO and founder, James Steyer.

The Campaign for a Commercial-Free Childhood said children under 13 are too young to grapple with the complexities of online relationships. The group is also concerned about whether kids will understand things like online privacy, what's appropriate to share, and who might have access to their conversations, pictures and videos.

The Campaign for a Commercial-Free Childhood has previously criticized other kid-focused tech. In 2013, it complained about claims made by Fisher-Price and Disney's "Baby Einstein" brand that mobile apps could be educational for babies. In 2015, it criticized Google's YouTube Kids app. And last year, it joined a coalition of advocacy groups protesting a Mattel-made artificial-intelligence-powered home hub aimed at kids.

On Tuesday, the group called Facebook "especially irresponsible" for encouraging children as young as preschool age to begin a social-media habit. While Facebook has said that the app is a way to talk to family and friends over long distances, the advocacy group noted such communication doesn't require a Messenger Kids account.

Kids can use parents' accounts on Facebook, Skype or other services to chat with relatives, the group noted. "They can also just pick up a phone," the group wrote.

"Raising children in our new digital age is difficult enough," it added. "We ask that you do not use Facebook's enormous reach and influence to make it even harder."

First published Jan. 30, 7:11 a.m. PT.

Update, 8:09 a.m.: Adds comment from Facebook Messenger; 11:46 a.m.: Adds comment from Common Sense Media, and additional background.

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