I'm curious about the difference between free speech and the right to a megaphone. From what I've gathered, the problem isn't that social media publishes harmf

I'm curious about the difference between free speech and the right to a megaphone. From what I've gathered, the problem isn't that social media publishes harmful posts at all, but rather that social media promotes bad posts to tens of millions of people. Isn't that a difference that regulation could rest upon?