I'm not quite sure I understand your analogy because I think most would agree that knives should not be regulated and we shouldn't investigate and punish knife sellers/makers for stabbings.
To play along with your analogy: if social media sites are well-designed, they do what they're supposed to do: communicate information or misinformation. And just like how we don't crack down on knives even though there are plenty of stabbings, we shouldn't be cracking down on social media & online speech even though there are plenty of liars and idiots who spread misinformation.
There is much more limited harm from a person wielding a knife for malicious purposes, as opposed to how quickly social media can fuel similar actions. If you genuinely believe scale doesn't matter and everything is the same as everything else, I don't know what to tell you.
I don't doubt that scale matters, it's just a bad analogy. Knives aren't comparable to social media, but if the commenter wants to play the knife analogy game then this is the conclusion.
But the object of knives isn't to stab people to death; a vanishing number of knives bought are used to stab people to death, and "lunatics" are not a core base for any knife seller.
A well-designed system does what it's supposed to. Facebook's either supposed to be addictive, or it just is by accident. If it's by accident, it's poorly designed. If it's not, it has an objectionable purpose.
To play along with your analogy: if social media sites are well-designed, they do what they're supposed to do: communicate information or misinformation. And just like how we don't crack down on knives even though there are plenty of stabbings, we shouldn't be cracking down on social media & online speech even though there are plenty of liars and idiots who spread misinformation.