"We have no real understanding of what the scope of misinformation is"
What a load of crap. Politics are covered by a wide range of media instruments: local and national TV news, print media, international journalists, video recordings, audio recordings etc. You bloody well can check almost all news as to whether it's "fake" or not if you bother one tiny bit to actually verify it with sources known to be accurate. And by "known to be accurate" I mean sources that consistently show high standards of accuracy. They do exist and are pretty easy to identify. The problem with these social media giants is the same problem with individual people: when you've made up your mind about a thing there's no desire or motivation to check that it's true. In the case of these companies it's also greed. They'll take anyone's money and look the other way when it comes to their associations. It's intellectual laziness and avarice on a corporate scale.