I don't think that social media alone is to blame, nor do I think it is the main culprit, it is an accelerant, but not the cause.
In order for fringe view points to be broadly accepted they need to be "approved"/"verified" by a trusted party.
Now that our news channels, like Fox News, and CNN, spend more time talking about their opinions of the news rather than what occurred, every fringe viewpoint has been stamped as "valid" by appearing on these "news" organizations.
Social media I think exacerbated this problem by reducing revenues for news media companies so they all had to focus on content that was more emotional, rather than informative. And if they can stir up an emotion people tune in.
Then those viewpoints are shared across social media to create a larger audience, again linking back to these "verified" sources, and you get a pretty vicious flywheel effect.