Around 200,000 accounts were deactivated by Wednesday on TikTok alone, with "hundreds of thousands" more to be blocked in the next few days after Australia banned social media for children under 16.
Many of the estimated 1 million children affected by the legislation posted goodbye messages on social media, according to Reuters.
"No more social media ... no more contact with the rest of the world," one teen wrote on TikTok. "#seeyouwhenim16," said another. Others said they would learn how to get round the ban.
"It's just kind of pointless, we're just going to create new ways to get on these platforms, so what's the point," said 14-year-old Claire Ni.
Australia on Wednesday became the first country to ban social media for children under 16, blocking access in a move welcomed by many parents and child advocates but criticised by major technology companies and free-speech advocates.
Starting at midnight (1300 GMT on Tuesday), 10 of the largest platforms including TikTok, Google, YouTube, Instagram and Facebook were ordered to block children or face fines of up to A$49.5 million ($33 million) under the new law, which is being closely watched by regulators worldwide.
Prime Minister Anthony Albanese called it "a proud day" for families and cast the law as proof that policymakers can curb online harms that have outpaced traditional safeguards.
"This will make an enormous difference. It is one of the biggest social and cultural changes that our nation has faced," Albanese told a news conference on Wednesday.
"It's a profound reform which will continue to reverberate around the world."
In a video message, Albanese urged children to "start a new sport, new instrument, or read that book that has been sitting there for some time on your shelf," ahead of Australia's summer school break starting later this month.
