This story about researches trying to dig into the effects of such fake stories I think highlights the point:
Hunt Allcott of New York University and Matthew Gentzkow of Stanford commissioned a survey in late November hoping to discern just how deeply some of the fake news embedded itself with American voters. The two asked people, among other things, whether they had heard various pieces of news that reflected positively or negatively on one of the candidates — of three varieties.
There was completely true news: Hillary Clinton called some Trump supporters a “basket of deplorables,” for example, or Mr. Trump refused to say at a debate whether he would concede the election if he lost.
There was fake news, as identified by fact-checking sites like Snopes and PolitiFact — big things like the Pope Francis story and smaller items, like Mr. Trump threatening to deport the “Hamilton” creator Lin-Manuel Miranda to Puerto Rico.
The third category was most interesting. The researchers created “fake fake” news. That is, they invented some headlines that were the type of thing fake sites produce, but had never actually been published during the campaign. One of these placebo headlines was that “leaked documents reveal that the Clinton campaign planned a scheme to offer to drive Republican voters to the polls but then take them to the wrong place,” and its inverse in which it was the Trump campaign scheming to take Democrats to the wrong polling place.
There is some good news in that more people reported having heard, and believed, the true statements than the false statements. Only 15.3 percent of the population recalled seeing the fake news stories, and 7.9 percent recalled seeing them and believing them.
The more interesting result: Those numbers are nearly identical to the proportion who reported seeing (14.1 percent) and believing (8.3 percent) the placebos, the “fake fake” news stories. In other words, as many people recalled seeing and believing fake news that had been published and distributed through social media as recalled seeing fake news that had never existed and was purely an invention of researchers.
What I think this shows is that in many ways "fake news" is more a reflection of pre-existing views and the way that people are interacting with news in the social media world than it is a true changer of opinion. In this sense, fake news may be responsible for increasing partisanship, but I don't think it's likely to have changed many minds.
The really interesting thing here, to my mind, is the way that people are interacting with news (fake and real) in a world of social media. Traditional, passive forms of receiving news (the nightly news, news magazines, news papers) are all on the wane. People see much of their news on their phones and computers, and via social media sites in which they see a feed consisting of links, capsule summaries, and comments. A news story gets wide play not simply by being in a major venue (say, being on the front page of a big city news paper, or being in the first ten minutes of the nightly news) but when lots of people make a point of passing the news on to others: sharing it, re-tweeting it, etc.
What news do people pass on? In the human realm, things that pull at the emotions: Isn't this terrible! This inspired me! I'm so excited to see this!
But in the political realm, a key reason people share news is because it fits into some existing set of beliefs and opinions they have. A story about someone finding tens of thousands of fake Hillary votes in an Ohio warehouse fit into a narrative that some people already wanted to believe that Democrats were dirty tricksters who were going to try to steal the election. A story claiming that Trump's treasury pick owned a bank that foreclosed on a 90-year-old woman over a $0.27 underpayment fit into a narrative that bankers are evil and Trump is aligned with unscrupulous people. A story claiming that a judge has ruled doctors can refuse to treat women who have had abortions or people who have had sex change operations fits a narrative that people who ask for religious conscience protection simply want to be mean to others.
One of these stories is totally made up, the other two are based on massive mis-reporting and selective reporting of the facts (which ends up with the stories as reported being basically false) but the way that people interact with them is the same. For instance, the last of these got wide play online because Star Trek actor George Takei shared it with his massive audience of Facebook followers. Many people probably only read the headline or the summary online before sharing it around, making comments on it, etc. Fewer still went so far as to click the link to the ruling (which turns out not to be a ruling at all, but rather the granting of a preliminary injunction) and discover that the articles being passed around on the topic totally misrepresented the point at issue and the nature of the judge's action.
To my mind, that fact that a news story can become just as "viral" when it's totally made up (indeed, sometimes more so, because then the story can totally fit the mood of the intended audience) is in a sense a symptom. For many people, the news is not a way to find out what is going on in the world, but rather a way to confirm the ideas they already have about the world. This, combined with the incredible speed of social media -- where a satisfying bit of "news" can travel the world in seconds and a less satisfying clarification never get spread at all -- is going to result in people reading a lot of self-confirming and often biased/incorrect "news" whether the stories are consciously made up or not.