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.
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.