You really don't know, do you?
> Following the 2016 election, a specific concern has been the effect of false stories—“fake news,” as it has been dubbed—circulated on social media. Recent evidence shows that: 1) 62 percent of US adults get news on social media (Gottfried and Shearer 2016); 2) the most popular fake news stories were more widely shared on Facebook than the most popular mainstream news stories (Silverman 2016); 3) many people who see fake news stories report that they believe them (Silverman and Singer-Vine 2016); and 4) the most discussed fake news stories tended to favor Donald Trump over Hillary Clinton (Silverman 2016). Putting these facts together, a number of commentators have suggested that Donald Trump would not have been elected president were it not for the influence of fake news (for examples, see Parkinson 2016; Read 2016; Dewey 2016).
Major news outlets, no. Not so major outlets, sure.
All the Googling I've seen seems to point to a "Santa Monica Observer" as the primary source of this claim. (See the Snopes article: https://www.snopes.com/texas-samir-al-hajeed-sam-hyde/ ), Snopes also links to a "Freedom Daily" and there are a couple of other sketchy news websites with this claim.
The Snopes article also mentions that a comedian known for trolling-style pranks (Sam Hyde -- https://en.wikipedia.org/wiki/Sam_Hyde#Hoaxes_and_pranks ) is tangent to these claims. So my best guess is that you are correct, someone treated a trollish tweet or post as actual news.
I'm not sure why bias is brought up all the time in "fake news", because bias is not the same as fake. Nobody should have issues with conservative news that is known for better vetting, even if their bias is strong (your Wall Street Journals, National Review, etc.). Fake is fake whether it comes from the right or the left. It is true that conservative fake news was much more prevalent in the 2016 election (https://web.stanford.edu/~gentzkow/research/fakenews.pdf), but "progressive" fake news definitely exists.
I don't think this is it. The very concept of fake news is, fundamentally, propaganda.
The mainstream media's privileged position of controlling the narrative is under threat, both from within the media (e.g. Breitbart) and without (random guys on the internet creating narratives).
The idea of "fake news" is propaganda, created in order to push Facebook/Twitter/etc into shutting down the media and random guys that are threatening the mainstream media. It'll probably work too.
Incidentally, insofar as "fake news" is real, it is highly unlikely to have any major effect. ...for fake news to have changed the outcome of the election, a single fake article would need to have had the same persuasive effect as 36 television campaign ads. https://web.stanford.edu/~gentzkow/research/fakenews.pdf