The Trump campaign used social media, outlets and alt right media, to exploit cultural issues in its political campaign. Russian advertisers used the social media to exploit the same cultural divisions. They were also effectively targeted to swing states in the election. Clinton was heavily favored in the election because she was expected to win the electoral college votes of states like Wisconsin and Michigan which she lost to Trump by an extremely thin margin. She would have won the election if she and won the rust belt states.
Bernie Sanders beat Clinton in the Wisconsin and Michigan primaries. His populist message was will received in the rust belt. Russian ads in those states exploited the controversy made public by the hacks in the DNC website which showed that the DNC favored Clinton. The ads were not were not designed to promote Trump; they were designed to discourage suppress Democratic votes in the rust belt states. The turnout for Clinton in those states was well below the turnout for Obama in the 2012 election.
The Russian ads and the Trump campaigns social media ads also exploited divisive cultural issues. The list of these issues is well known. Trump is still running the same campaign from the White House. The Black Lives Matter issue was extensively used along with its counterpart Blue Lives Matter. Terrorism is a real concern for everyone but concerns but Trump and the Russians used ads to increase the fear of Islamic terrorism. Trump positioned himself as the superior protector from terrorists. Immigration issues dovetailed with terrorism fears, and the exploitation of white nationalism. They also worked well with Trump's promises to promote American jobs by erecting trade barriers.
This article also shows how the business models of Facebook and Twitter enable the exploitation of cultural issues at low cost. When a media issue goes viral millions of users receive a message that cost very little to sponsor. The cost per user is much lower than ads on TV which do not target a specific audience. The social media also have the advantage of anonymity. Unlike traditional media, in which ad sponsors are apparent, Facebook does not reveal ad sponsorship. US voters had no idea that Russian ads or Trump ads were sponsored by them. They even did a good job of creating individuals who did exist as sponsors of their ads.