I think it was the subversion of chains of trust by the kind of social media were people are linked to friends and family.

For example, say that you're connected to a cousin of yours in Facebook, somebody you know well as an excelent and honest person. Now, say they share this post about how lots of robberies in their neighbourhood have been traced back to youths in this other neighbourhood which is mainly of people from a different etnic group.

Do you believe it? It comes from somebody who is an excellent and honest person, so you know they wouldn't lie about something like this.

At this point, anybody who actually thinks properly about the quality of information has figured out that just because somebody is honest doesn't mean they tell the truth: they might be telling you something they genuinelly believe is true when it's not because they themselves were decidved into thinking of it as true, whilst its not - in other word and simplifying it a bit, you can't trust information from an honest person if they're gullible, or even if they're just prone to "spread the word" without having tought about it first (and the way things like Facebook are designed is exactly to induce people to share in response to an emotional reaction and without thinking)

How many people do you know that really figure it out before sharing, or even worwe, did so back in the day before fake-news became a widelly talked about phenomenon?

Add to this subversion of chains of trust that the fakest stuff is usually the most shocking and thus most likelly to be shared and you have this great way to spread misinformation in such a way that it keeps getting whitewashed as ot spreads thanks to preexisting trust in friends and family.