It doesn't need to be true, it just needs to be shocking, and people will promote it.
This is only fixed by good moderation, with prominent (and powerful) "flag" buttons like Hacker News has. Hacker News and Reddit don't suffer from nearly as much fake news, because moderation teams tend to do a good job of weeding out garbage, and have an easier time with the prominent report buttons.
Facebook and social networks have absolutely no moderation.