Unequal access to wide dissemination is already a given btw. It depends in part on how much money you have.
Note: I'm not talking about censorship of individual posts they make for the views expressed in those posts. That's what Facebook was doing here, and I fully support that. (I'm from Canada; the concept is enshrined in law here!)
I'm just talking about the idea that you probably don't want third parties like Facebook to be responsible for deciding whether to censor people, rather the much "safer" question of whether to censor individual posts. Especially when those people are current political candidates, and the people calling for their de-platforming are people opposed to their election, who are perfectly willing to libel them if it silences them at a time crucial for swinging an election.
Facebook, unlike a government, does not have a concept of due process, and so is not equipped to deal with complaints from people who have motivations (like swinging an election) strong enough to be willing to "take it in the chin" for libelling someone, as long as their libel causes the desired de-platforming effect at just the right time.
This is why we tend to want censorship to be a legal matter: courts have well-built, well-thought-out mechanisms to allow the recipient of a complaint against them to fight back. Meanwhile, what kind of arbitration system does Facebook have? Or, more abstractly, what kind of arbitration system are they required to have? Are there any requirements at all, for how Facebook and other networks should work with people, when others are asking for them to be de-platformed?
In the case of a whole party/movement, I would say it's a really fine line to walk. I wouldn't advocate for Facebook to ban parties like the AfD in Germany or FN in France, while they're often extremist, there is still some legitimate opposition within the bounds of the constitutions left in those parties. But I wouldn't be opposed to FB banning e.g. the KKK or some party that explicitly made reference to Mein Kampf or to killing all the Jews or similar.
Also beware of the slippery slope fallacy. Just because one believes that deplatforming some parties or movements is legitimate, it doesn't mean nor necessarily lead to deplatforming all opposition.
Maybe not, but when it comes to politics, unequal access to wide dissemination can be harmful to the democratic process. For any given country, either all parties or no parties should be able to post to Facebook—but certainly not some parties but not others.