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.
Facebook is not designed to be addictive, it's designed to be engaging. Lunatics aren't the core base for Facebook either, a small minority of Facebook's total user base cause problems. It looks like you've already made up your mind that Facebook is addictive and bad, you barely have to dress it up with this analogy.
There's no solid evidence that it is addictive except in a colloquial sense. Addictive substances like opiates, nicotine etc consistently induce an addiction response in humans with very predictable outcomes. Facebook isn't anything like that, but its detractors love to use the word "addiction" to imply a medical phenomenon that doesn't happen. The "harm" of Facebook usage is not even close to the reliably brutal consequences of actual addictive substance abuse, nor is it induced with any consistency. Unlike actual addictive substances, Facebook does not alter one's ability to inhibit their own behavior.
There's no doubt that Facebook can negatively impact one's self-esteem and life, but that's like any entertainment. Even alcohol addiction is better understood and arguably more brutal, yet the only restriction on alcohol is an age limit.
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.