If Facebook didn't have a monopoly on social media, would competitive pressure make it stop what you consider misinformation? Of course not. You can find a stream of misinformation in more marginal press outlets. Only larger outlets that have a relative monopoly (NYtimes say), can afford to be relatively free of crude misinformation (but even here they have their very definite spin as does everything).
The only reason people are talking about forcing Facebook to impose some restrictions on communication is because Facebook has somewhat of a monopoly on some communication.
Which is to say that talk of Facebook and regulation winds up being "Facebook is big and powerful and we want the state to have that power instead, damn it". Without liking Facebook's approach all that much, I'm less of a fan of this.