It does strike me that the polarization is more of an internet phenomenon rather than a social media phenomenon (although of course they are tightly linked). People get most of their news from the internet now, and to cope with the disruption major outlets started having to make a lot more clickbait and inflammatory content, and that results in polarization.

I think these charts say it all: https://marginalrevolution.com/marginalrevolution/2019/06/th...

It was still social media that made it happen. Clickbait is needed because you're part of the social media feeds and have to compete with other clickbait. Without a common feed, you don't need a bait. User has already arrived at your webpage.

I thought it started during 2016

At some point, the question needs to encompass all social media platforms. Arguably, Facebook's days of relevance are over, and we need to be looking where the eyeballs actually are.

They have 3 billion monthly active users. On Facebook, not IG or WhatsApp.

At minimum, they sure as hell sold the shovels.