As you say 'social media' is not a good category, we should specify exactly the things that are concerning. Here are the ones where I'm concerned about their effect on young people:

1. a user is shown new content based on extensive profiling and a secret algorithm that the user does not control

2. a users activity can be discovered and tracked by people that intend to take advantage of the user

3. the operation of the site is optimised for addiction (or more euphemistically "attention")

I absolutely don't think that a book club or a kids own website comments or person to person chat systems should be included in the rules.

Note - I'm not saying these things should be banned, just that I think it's reasonable to restrict their use to adults.