When a given company represents 90% of the daily information stream of your average citizen, it is a monopoly.
That may be relevant, if it were to try to use that monopoly status to get into a different market. Facebook is not doing that, so let them be. The barrier to entry into their market is none-existent — various snapchats, instagrams, et al. have done that. Facebook itself unseated MySpace in front of our eyes.
In the meantime, we need a way to ensure that citizens actually get all information that is relevant to their vote
Who are these omniscient benevolent "we", that need to ensure something for the "citizens", god bless their pretty little heads? No, the "we" and the "citizens" are the same people — and your sentence makes no sense.
A pragmatist would also acknowledge that making the public more informed is more important than giving FB freedom to censor whatever they want.
So, your proposal is to surrender an essential liberty in exchange for a hypothetical temporary gain?.. Don't we already know something about this approach?
The outrage is not that Facebook is censoring whatever they please. The outrage is that others aren't afforded the same liberty.