I have had a lot of similar thoughts recently. If you think of this historically -- soon after mass-communication technologies were first invented, they were eventually abused, the radio propaganda of WWII being one of the best examples.
Laws were introduced, and corporations grew up around these technologies, the producers of the content became professionalized, and by doing it full time, some of them even developed ethics and standards of behavior.
Now, that old media has been destroyed by new technology. The new technology is fundamentally superior in terms of the volume of information that can be transmitted, and it is both push and pull. However, the old media's "checks and balances" were also destroyed, and that is having incredibly negative consequences.
I think there are some concrete, simple, technical steps that Facebook & browser vendors can do:
* Before allowing you to share an article from a source that is of questionable trustworthiness, it should show you a warning message: "This site is known to contain untrue content." They might also consider adding an alert next to links from these sites when they are being displayed in the news feed. I think technically, this problem is not that different from the way that email systems deal with spam.
* Similarly, the way that browser vendors show you an alert before allowing you to navigate to sites that contain viruses, etc, the browser can show you an alert message before taking you to a site that is known to have untrue content. And just like you see a red error message when visiting a site that doesn't have a proper SSL setup, this could appear for sites that are untrustworthy.
Don't underestimate the psychological power of strategically placed small red warning messages over time.
Obviously making the decisions of what goes on the list will be a highly political affair, but what is nice to realize is that, unlike the government, all these technologies are developed by private companies and given to their users for free. If you keep the bar for what is considered "lying" pretty high, most educated professional people (who are the employees of these companies) would be able to agree on which things to warn about.
Additionally, the "social media" space has greatly matured and consolidated. There is no longer really a direct competitor to Facebook. Facebook & browser vendors now have the opportunity and, the responsibility to innovate in this area. I think they can do a lot without really impacting their bottom line.
Don't allow yourself to get sucked into the problem of how computers might be able to determine "truth" -- that, is absolutely unsolvable. We will not be able to censor the CNN, the NYTimes or even Fox News (in general). However, there are many dark corners of the internet that are actually pretty influential, that could be toned down with this approach.