Strictly speaking, only those aged 13 and over are allowed to use Facebook. But the prevention methods are trivial, meaning more than 20 million under-13-year-olds are thought to be using the network.
Facebook on Monday launched its first app tailored for young users.It's a ringfenced network that needs parental approval before use, and will not - the company has promised - be used to feed data for advertising.
Messenger Kids is a simplified, locked-down version of the messaging app Facebook today offers those over 13.
There will also be "a library of kid-appropriate and specially chosen GIFs, frames, stickers, masks and drawing tools lets them decorate content and express their personalities".
Approved adults can also contact children through the app - although they will get their messages through the normal Facebook Messenger app.
Messenger Kids will of course collect data: the child's name, the content of the messages, and typical usage reports for how the app is used.
Facebook will share that information with third parties, which must have data protection policies that comply with Coppa, the Children's Online Privacy Protection Act in the US.
Facebook has promised the data will not be used in any way to power the "grown up" Facebook.
The app doesn't know specifically how old the children signing up are, so users will not be prompted to move onto Facebook when they are old enough.
If a child does decide to join full Facebook, it will be a brand new account with no data carried over from what was said on Messenger Kids, reports BBC.