Facebook has been used to incite genocide: https://www.nytimes.com/2018/10/15/technology/myanmar-facebo...
and enabling the spread of anti-anti-COVID stuff likely has a decently large body count, as well.
The difference between humans pre and post social media is that the we have a platform now that amplifies many voices.
We also treat it as a news source - and as we know, bad news has always outsold light fluffy good news when it comes to competing for our attention.
Couple that with the issue that engagement (comments, likes, shares) increase how visible a post is and you get the recipe for how to rapidly help polarize a society.
This is of course to say that we were already polarized and already arguing with each other, but now we can do it with more reach and with a louder voice.