It's also illegal to partake in cigarettes in certain areas, even for adults. Cigarette packaging and location is also regulated. It's not like adults have unfettered access to things that harm them. It's illegal to drink and drive. In some places, it's illegal to drink in public, or it's illegal to sell alcohol beyond certain time periods. I'm not saying that these things are right, but that if we're going to compare social media to harmful substances we shouldn't misrepresent the access to harmful substances adults actually have.

I would suggest analogous rules should be determined for social media.

I am not particularly confident what such rules should look like — I don't think there's an easy equivalent to drunk driving, but perhaps "smoking in a confined public area" might be close to "trolling on forums that people need to use to get normal life activities done"?

But they are their own thing, and need to be treated appropriately.

All of your examples, like drunk driving and indoor smoking bans, exist because those activities are likely to harm others.