I think the addicted problem is the user's problem, not the product problem, like killing is human's problem, not the knife's problem.
If social media sites are well-designed, what does that say about their 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.
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.
There’s an old saying that I absolutely love: I am only responsible for what I say, not for what you understand.