Welcome to insanity.
When a given company represents 90% of the daily information stream of your average citizen, it is a monopoly. Any attempt to challenge that will have to run against an extremely high barrier to entry established by said monopoly. It doesn't mean that it can't be unseated - but doing so requires immense resources, and even then would take many years.
In the meantime, we need a way to ensure that citizens actually get all information that is relevant to their vote, rather than the one that our monopolist decided to tell them. An idealistic libertarian would say that, by choosing FB, they implicitly give permission for such screening. A pragmatist would acknowledge that vast majority of FB users didn't actually think about it at all, and didn't realize that they're setting themselves up for an information bubble. A pragmatist would also acknowledge that making the public more informed is more important than giving FB freedom to censor whatever they want.