The EU on Thursday opened a formal investigation into Facebook and Instagram on suspicion the platforms owned by Meta are causing addictive behaviour in children.
The probe is under a mammoth law known as the Digital Services Act (DSA) that forces the world's largest tech firms to do more to protect European users online and clamp down on illegal content.
It is the second investigation into Meta after an earlier one launched by the European Union last month over fears Facebook and Instagram are failing to counter disinformation.
In Thursday's announcement, the European Commission, the EU's tech regulator, said it suspected the platforms' systems "may stimulate behavioural addictions in children".
Another issue the commission raised is the so-called "rabbit hole" effect -- which occurs when users are fed related content based on an algorithm, in some cases leading to more dangerous content.
"We are not convinced that it has done enough to comply with the DSA obligations to mitigate the risks of negative effects to the physical and mental health of young Europeans," the EU's internal market commissioner, Thierry Breton, said of Meta.
"We are sparing no effort to protect our children," he added.
The commission is also worried that Meta's age-verification tools may not be "proportionate and effective".
The DSA has strict rules to protect children and ensure their privacy and security online, and the EU fears Meta might not be doing enough to tackle these obligations.
The EU stressed in a statement that the "opening of formal proceedings does not prejudge its outcome".
The DSA is one law among many in the EU's powerful armoury to rein in Big Tech.
Facebook and Instagram are among 23 "very large" online platforms that must comply with the DSA or risk fines that could reach as high as six percent of a platform's global turnover, or even a ban for serious and repeated violations.
Other platforms include Snapchat, TikTok and YouTube.
Brussels has launched a wave of investigations, showing online giants it means business.
In February, the commission began a probe into TikTok, which is owned by Chinese firm Bytedance, on suspicion the hugely popular video-sharing app may not be doing enough to address negative impacts on young people.
The EU also forced TikTok to suspend its spinoff Lite app's reward schemes in April after warning its "addictive" nature could risk serious damage to users' mental health.
Other investigations have targeted Chinese online retailer AliExpress and social media platform X, which is owned by tech billionaire Elon Musk and used to be called Twitter.
The DSA's remit is wide and also forces digital shopping platform like AliExpress and Amazon to do more to counter the sale of fake and illegal goods online.
