Mass media did it from the start, but was less effective because they were trying to create "one size fits all" narratives that could reach and influence very large portions of the population. Recommendation engines tuned to the individual changed the game dramatically. They can lead people down rabbit holes that the other people in their life don't even know to exist. At least when the town newspaper or local church starts shilling some truly wacky shit, everybody in town knows about it. But now youtube/etc personalized recommendation engines can send your brother or father down radicalization paths you never even heard of before and had no possible way of noticing before the damage was done.

If you think "Oh I'll just watch youtube to get an idea of what other people are watching", you're stuck in 20th century thinking. It doesn't work like that anymore, youtube/etc are fundamentally unlike old media. These systems have atomized society in a way that newspapers never could.