How news stories enter our (online) lives has changed radically, but we're still depending on assumptions about how old (print) news filtered truth from fiction. IOW, we're assuming other people (or search tools) are filtering for credibility. THEY AREN'T.
1. Twitter, Instagram, Snapchat, and Facebook have all made it insanely easy to share linked content with huge numbers of people, quickly, which means lies spread faster than truth.
2. The generic Facebook "shell" for shared articles and Google's "AMP" news carousel make it hard to distinguish between an article in the NY Times and an article fabricated by a 20-something on a couch with a profitable clickbait business. Most readers fail to look at the url. If your friends on social media are sharing questionable sources, don't just blast them publicly with accusations; take it offline, empathize with their values, and then talk about sources.
3. There are substantial (and substantiated) allegations that foreign news agencies are intentionally producing misleading stories (video) targeting Americans in order to destabilize Americans' trust both in news and in civic institutions. They use targeted marketing techniques to place articles as promoted content in the feeds of users likely to share the content. (This is in addition to allegations about Russian hacking.)
4. Many people are unfamiliar with Google's ranking algorithms, and believe that articles or sites that appear higher in search results are more credible, when Google does not rank or filter for credibility. This misplaced trust in Google may have helped Dylann Roof become a racist (video). IOW, to the extent that people attempt to evaluate credibility, they often use flawed reasons for deciding a site is credible. Once again for those in the back row: GOOGLE DOES NOT RANK OR FILTER FOR CREDIBILITY.
5. Though the web's democratizing potential is huge, so is its tendency to reinforce political polarization. When we surround ourselves with like-minded people, one of the main purposes of sharing articles is to affirm group identity. In other words, we share not in order to inform, but in order to say, "I'm like you and you're like me." When that's the purpose, truth and accuracy take a back seat and let ideology drive.
6. Many people have lost trust in "mainstream media." This lost trust often takes the form of "Mainstream media isn't covering this story." Those claims are often unfounded and lazy: often, a quick Google news search reveals that mainstream news is covering it, but perhaps not with the biases some might prefer.
7. And sometimes, the story isn't being covered nationally. Why? In short, the internet has hit most news organizations' bottom lines. Thousands of journalists are out of work as a result. With limited staffs, news outlets don't have the resources to send journalists to far-flung places or to verify the story and are therefore hesitant to cover stories outside their local area. According to this entertaining but accurate segment by John Oliver, many newspapers have even cut coverage of local politics. Solution? Pay for a subscription or two. Blogs, twitter, news aggregators and "citizen news" sites are no replacement for the resources of a fully-staffed news desk with investigative reporters and fact-checkers.