I don't know to what extent "fake news" influenced the latest election. I certainly know that the Democrats would have us believe that it was a major factor in their loss - which is the kind of self-serving non falsifiable assertion I'd expect from them in the circumstances, just as the Republicans have always made similar claims about media bias when they have lost elections.
As Pete points out, there is no patent on "fake news". The two phenomena described on this thread are different, but both equally consistent with the label applied. The form that the Democrats are most concerned about (which I see as the lesser of the two) is certainly easier to identify - which is why it's less insidious than more subtle (but widespread) methods of propagnda. While this form may have been used more against their party in this particular election cycle, I expect it to be used by both sides in the next cycle and for it to equalize and become a general propaganda tactic for both right and left sources in the coming years. When you consider the nature of social media and the way "news" spreads on those platforms within ideologically homogenous and closed off groups, it becomes something of an inevitability I think. Maybe the right got there first this time around, but I guarantee you the other side will catch up.
Certainly, Facebook and other networks already excel at delivering customized feeds of "news" to their members designed to fulfill those users' individual preferences. It's alot like what the newspapers have done (feeding their ideological niches) but at 10x the efficiency. In a newspaper, even one that fits your ideological preference, there is always the danger that you may run into something contrary or unappealing to your world view. This becomes far less likely to happen through a personalized social media feed.
It's one of the things I absolutely loathe about social media in general. When you talk about the toxic demagoguery infecting our political system (and here in Canada we are not immune) I have no doubt that social media is accelerating the process. I consider the decision to delete Facebook one of the best ones I have ever made.