Considering "fake news" (literally commandeering the media and planting untrue stories) was a tactic used by Hitler and is considered to be a basic tool of authoritarianism, I'd suggest that no one has underestimated its impact.

In fact, fake news started WWI.

> literally commandeering the media and planting untrue stories

We assumed that only happened with state-controlled media, that one would first take control of the State, before taking over the media to sediment it. We weren't prepared to deal with the Murdochs taking control of the State by controlling part of the media.