FB has almost every eyeball now. Can you say the same about your fake Necronomicon? Book of spells? The media that controlled your mind?
The last statement is basically one against any form of control or regulation. If people use the x to make themselves worse that's a failure of people not x. Sometimes people need help.
They are a platform rather than a publisher, in my view at least.
For all intents and purposes they should be responsible for everything that happens on the platform regardless of where the content was picked up from. They should have a responsibility but this comes in conflict with their goal to to drive engagement and profits.
Their algorithm curates what people see, shows specific links, and influences opinions. And when you can influence opinions on such a large scale and monetize it we have a conflict of interest and even a weapon. FB and others have proven this in the past. Why do you think the media is regulated so tightly around election time? FB can take even a blog post and push it so aggressively right before elections that they manage to sway opinions but still claim they were just a platform.
Let's put it in terms other than politics...
In the early 1980s a full third of all households in the US watched the TV show "Dallas" every weekend. Was "Dallas" the greatest performance ever made or just the best option out of 3 choices?
I wonder what the cell phone generation would think of our pulp paper fake Necronomicons from the 1970s? There was also a book of spells at my local library in the very-white very-evangelical suburbs of Little Rock, AR when I was a kid, too. I liked the one about how to become a werewolf a lot, but didn't really get to the point of going and murdering a dog and painting myself with its blood and boiled fat.
The pearl-clutching about social media is because power has been taken away. Media executives can no longer fancy themselves in control of people's minds, because they no longer have a monopoly on eyeballs via print and TV information.
Worse yet (for them), the user customizable nature of social media feeds mean that they can't even know what people really see.
If people use the internet to make themselves worse that's a failure of people not the internet.