Facebooks role? As Facebook user you reveal all kinds of preferences. That is used to target ads. Now the GOP bought targeted Facebook ads, targeting very specifically probable opponent supporters. They fed them with with semi-truths (aka fake news) to keep them from the voting booth or do some “protest vote”.
Likewise the GOP used (non-Facebook) information to identify potential GOP supporters and rang their doorbells to affirm to you personally that “every vote counts”, with an app that coordinated supporters where to ring the door bell.
The Cambridge Analytics CEO revealed this two-pronged strategy in a talk about “The Power of Big Data and Psychographics”: a) ring at the doorbell of supporters and b) place destabilizing content in the Facebook stream of opponents, as identified by Facebook likes and advertisement customer profile.
It’s not so much about individual people sharing stupid stuff — it’s about injecting politically motivated content into your stream as “sponsored content”.