Fake news about Hillary Clinton is a good example, as is Breitbart's very existence, as are foreign reddit bots, as are a lot of political extremists parroting views in their own posts on subjects about which they have no knowledge, etc.
Granted, for things like Breitbart, one has to affirmatively opt-in.
Granted, for things like Breitbart, one has to affirmatively opt-in.