There is no evidence for these claims about massive effects of online ads and lots of reasons to be skeptical - see my @UpshotNYT on fake news https://www.nytimes.com/2018/02/13/upshot/fake-news-and-bots-may-be-worrisome-but-their-political-power-is-overblown.html … and the primary study on effects of microtargeting is deeply flawed http://www.pnas.org/content/early/2018/05/16/1805363115 …https://twitter.com/jon_m_rob/status/1031284297142030336 …
Agree that one time impressions not very important but ongoing exposure/community formation surely has some effect through socialization. Yes, we’ve had this disagreement often. :-)
US political public sphere norms clearly shifted—somewhat rapidly. All this is one vector.