>even if you were in a private browser

Incognito mode is about not saving data or browser history to your computer. Sites can still identify you if you login or even just from your IP. It's not meant to make you anonymous. This is a common misconception which is why these modes show a big warning explaination when you enable them.

>they'd go to prison

That's for the courts to decide. The Facebook and Instagram apps may have already gotten consent from the user to share this information.