How "fake news" exploits Facebook's preference for engagement

There's a catch to optimising for engagement, despite all the benefits above.

If something is controversial, or sensational for the wrong reasons, this can get unnaturally high engagement and be rewarded with cheaper delivery and therefore greater reach too.

It's no wonder "fake news" does so well on Facebook, and hard to suggest how Facebook can counter this without removing the aforementioned benefits to both advertisers and end users. I'll leave it to Facebook to comment and act upon this systemic issue.