The feedback loop is much tighter now. The more outrageous a story is, the more people click it, and the more money that story makes. You can immediately measure the effect (the Macedonian teenagers quickly zoomed in to the more gullible Trump supporters, because they made more money).

In the past news sources had a reputation to protect, so crazy stories were confined to tabloids and some radio shows. Now all these stories just pop up in peoples streams, without anything to distinguish them from stories from reliable sources.

Couple that with the fact that, even with trustworthy sources, you can paint an inaccurate picture just by editing which stories you present (which Facebook is incentivised to do and actually does), and I think the problem is much worse now.