It is in the court of public opinion, which is where this battle is being fought right now.
Anyway, I'm not a lawyer but when I was in various business classes in college, they talked about cases where unions argued that certain company practices that had been long-standing (such as giving workers turkeys at Christmastime), could not be arbitrarily stopped because they had been so longstanding as to have become de facto policy.
I doubt that was a legally binding thing or just an arbitration thing, but as with concepts like common law spouses, are their some business practices that become de facto contracts after being in force for so long?
In Facebook's case, it was not their practice to ban content until conservatives won the big elections, which makes Facebook's action seem arbitrary given that they wouldn't be doing this if the election went the other way.
An arbitrary contract is no contract at all. I believe the legal practice is that an arbitrary or ambiguous contract term (like "hate speech" or "fake news") is to be decided against the party that drafted the contract, which would be Facebook.
-PJ
The Lefties would want to keep defeated Deplorables on the sites to mock them daily for fun.