As long pointed out here, the problem is that frictionless sharing platforms enable rapid dissemination of propaganda messaging, often intended to sell eye balls to advertisers. When anyone can post stuff for free, then anyone will post stuff for free!
Facebook now agrees this is the crux of the problem:

“People are writing outlandish headlines so they can get clicks and can get ad money, so probably the most important thing we can do is go after the economic incentives,”

What they are doing about it:

And Zuckerberg said last month that the company “made progress demoting false news in News Feed, which typically reduces an article’s traffic by 80 percent and destroys the economic incentives that most spammers and troll farms have to generate these false articles.”

Wonder what the false positive rate is on the above filters? They claim they are catching 99% of bad content in some categories.

The company has more than doubled the number of workers it employs to filter content, to 14,000, and plans to have 20,000 dedicated to that task by January.

That starts to sound like China’s Internet censorship system. Hmmm.
The fundamental problem is selling ads, including targeted ads, next to unvetted user produced content – and unvetted content from professional fake news sources too.

Facebook believes that more software will solve the problems…
Source: Facebook sends video about fake news to ad clients