Why fake news is a tech problem

News used to be reliable.
While "fake news" has always existed, we enjoyed a halcyon period in the second half of the 20th Century where the news media in industrial democracies produced a reliable, shared reality. People disagreed, but their opinions were mostly based on credible facts produced by solid journalism standards, fact-checking and editorial integrity.
Then the web arrived, followed by the social web. Now, instead of three reputable news sources, you hear facts and ideas from thousa