> I have tried to find good scientific evidence that shows that social media is a net negative for kids and or adults. I have been unable to do so.

The author mentioned by GP is currently working on a similar questions collecting, reviewing and categorizing known literature in these open access documents [1][2]. I suggest you take a look if you are interested in the topic.

> For example, Hacker News is the only social media that I use, and I feel that I use it very differently than folks that use Instagram, for example. Can they be effectively conflated?

Well, I would say no. But to have a meaningful discussion we need to first agree on what is meant here with "social media". Clearly, this law has been passed with the intent to affect Meta / ByteDance / Reddit and similar companies with a business model that hinges on capturing as much attention of their users as possible, which is very different from HackerNews. Most accusations to social media begin bad are towards of the former type.

> but I would be very curious to see a scientific justification for banning it for kids under 16.

From [1], it seems to me that there is a non-negligible amount of literature that has been accumulating, that could be used to justify the ban. Though, Australia is not a technocracy (I hope), so I would say that there is also a certain degree of "purely social" reasons why they might want to curb the access of social media companies to their youth.

[1]: https://docs.google.com/document/d/1w-HOfseF2wF9YIpXwUUtP65-...

[2] : https://docs.google.com/document/d/1vVAtMCQnz8WVxtSNQev_e1cG...