Re: "they seem to have only targeted right-wing (presumably) Americans."
No, the trolls targeted both camps to stir division
And it's by no means limited to the US - look at Myanmar and the refugees or India and the huge surge in killings based on fake news spread by FB and/or Messenger.
In short, give fanatics of any stripe an easy (and cheap) propaganda platform and they'll use it, regardless of the consequence to the targets of their bile. Human nature sucks.
(This doesn't absolve FB/Google of the blame for aiding this and placing their income/share price over the cost to people and society)