Alexis Madrigal describes What Facebook Did to American Democracy and why it was so hard to see it coming. Foreign exploitation of Facebook's ad system in the 2016 election was just the end result of Facebook's filter bubbles and its wildly successful efforts to get media to fill them. tl;dr: the horse was already dead before Russia flogged it.
The information systems that people use to process news have been rerouted through Facebook, and in the process, mostly broken and hidden from view. It wasn't just liberal bias that kept the media from putting everything together. Much of the hundreds of millions of dollars that was spent during the election cycle came in the form of "dark ads."
The truth is that while many reporters knew some things that were going on on Facebook, no one knew everything that was going on on Facebook, not even Facebook.
Facebook's uncanny method is to trickle enough traffic to publishers so they chum it constantly with Facebookish content, but not so much that publishers can assimilate Facebook visitors into their own audience. Unfortunately for this clever and destructive arrangement, the new far-right sites represented such a cohesive emergent affinity group that Facebook's machinery was co-opted.
It's said (usually on Twitter) that no-one is better than Nazis at exploiting a libertarian dropout's ideological impostures. This sort of thing usually strikes me as pompous and vague, but Facebook so perfectly embodies it I'm going to need two leftist energy bars for breakfast this morning.