More than 100 child health advocates and 19 nonprofits signed a letter sent to Mark Zuckerberg on Tuesday, urging him to shut down Messenger Kids. The letter, organized by the Campaign for a Commercial-Free Childhood (CCFC), is the latest initiative by those who have denounced the app since its December release.

The letter cites research showing that excessive social media use in teens is linked to poor sleep habits, unhealthy body ideals and “significantly higher rates of depression.” Thus, it argues that it would be “irresponsible” to promote a social media product for children as young as 5 years old.

The message to Zuckerberg also stresses that young children are “simply not ready” to combat the addictive nature of social media. Shifting their friendships online at such an early age will impede their ability to “read human emotion, delay gratification, and engage with the physical world.”

While Facebook markets the app as a way to strengthen kids’ relationships with long-distance family members, the letter insists that connecting with relatives can be accomplished via Skype or a similar platform. It does not require Messenger Kids.

In response to criticisms, a Facebook spokesperson told Wired that, since the app’s unveiling, parents across the country have applauded Messenger Kids for helping them better connect with their children.

“For example, we’ve heard stories of parents working night shifts being able read bedtime stories to their children, and moms who travel for work getting daily updates from their kids while they’re away,” the statement reads.

The letter acknowledges Facebook’s claim that the app will offer a “safe alternative” to underaged kids already on social media, but maintains that “11- and 12-year-olds who currently use Snapchat, Instagram, or Facebook are unlikely to switch to an app that is clearly designed for younger children.”

Finally, the letter references some criticisms Facebook has faced in the last year and slams the company for failing to uphold its promise to “do better.”

“Raising children in our new digital age is difficult enough,” the letter states. “We ask that you do not use Facebook’s enormous reach and influence to make it even harder.”

The CCFC also launched a petition which has garnered nearly 1,700 signatures as of today.

Read the full letter here