Since Q&A websites like Answerbag were conceptualized.

About the same time people stopped reading books and started becoming dumb assholes.

What are you talking about? Ad Hominem was THE FIRST legitimate form of argument and debate. Every 5 year-old starts there! When Roman rhetoricians coined the term, they were putting it forward as HOW TO ARGUE, not as something that was out of bounds - though they did point out that you shouldn't be fooled by it yourself. It reamained central to all debates (the fun ones anyway) for centuries. Back in the 16th Century, it reached dizzying heights of popularity and artistry - nothing like the Reformation for bringing out people's passions and convictions. (One of my favorites is where "Saint" Sir Thomas Moore - in answering Luther's teaching on Grace, Salvation and Free Will, accuses Luther of performing oral sex on menstruating sheep. (That's one of the tasteful jabs.) I can't sanitize the things Luther said in return, so I won't try.) In the early US, that supposed golden age of American decency, rationalism, and political genius, the political campaigns of 1796-1824 were 100% about character assassination and vicious ridicule. It took slavery becoming the dividing and defining issue before politicians actually had anything substantitve to say -- check out the famed Lincoln-Douglass Debate (famous for its lacke of Ad Hominem), if you can stay awake through it.

1976?

The question assumes that they (argumentum ad hominems) are legitimate forms (that they ever became legitimate forms). Certainly, they are illogical and always will be. The argumentum ad personam is actually disrespectful and abusive. That is why it is commonly referred to as ad hominem abusive. They have been in common use since recorded history began, and most humans use them almost instinctively (although we may be socialized into them) from a very young age. This is a behavior that needs to be unlearned, although most never attempt it. They have been quite common out here on AB. Lately, I've been thinking of coining my own logical fallacy: argumentum ad changum topicum. :P

"An ad hominem argument, also known as argumentum ad hominem (Latin: "argument to the person", "argument against the man") consists of replying to an argument or factual claim by attacking or appealing to a characteristic or belief of the person making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim. The process of proving or disproving the claims is thereby subverted, and the argumentum ad hominem works to change the subject." Source and further information: http://en.wikipedia.org/wiki/Ad_hominem_fallacy This kind of fallacy has been used very often. Repeated use of a fallacious argument will never make it legitimate.