"The fact that Facebook could easily throw the election by selectively showing a Get Out the Vote reminder in certain counties of a swing state, for example, was a running joke."
I think they should be, once they get to the point where they are ubiquitous enough to be considered an official identity and information provider (in the way that newspapers and TV have been), something which they've brought upon themselves in advocating a "real name" policy.
Though of course it's a different thing when a county inadvertently affects vote results by unintentional bad design, versus when a centralized entity intends to have an effect.
Indeed. Zuck was forced to backtrack his denials following the election, and is now committed to tackling fake news. They sing a very different tune now.
The idea was telling only those voters in certain counties (who predictably would vote a certain way) to vote; i.e., a less-blatant version of telling exactly the subset of people who are registered to vote for a given party, to go out and vote. That is a partisan action.
The idea that UI/UX can change the outcome of an election, though, isn't terribly new. Consider 2000's butterfly ballots: http://www.asktog.com/columns/042ButterflyBallot.html
Though of course it's a different thing when a county inadvertently affects vote results by unintentional bad design, versus when a centralized entity intends to have an effect.