Agree that there can be obscenity laws vis a vis minors. But the whole point of your initial post was “ And why are we allowing FB or instagram to determine what IS the appropriate age to use social media? Isn’t that an issue that should be a matter of public policy”. My responses ( which jumps off with the point that for R and Pg….almost all of which are not obscene even under lower minor standards ….the government does not and cannot control things and no laws exist to do so) are meant to answer that question.
The reason is that except for obscenity it can’t be a matter of public policy under the law as it exists today. Importantly here the issues that are concerning people with social media are not obscenity related content. So regulation is very very difficult.