If Facebook wanted to be a left wing, liberal-progressive website, that censored conservative speech, they should have presented themselves as a left wing activist forum. They should have presented in their terms and conditions of use, as a censoring and banning conservative speech. They should have let the public, their members, stockholders and other regulatory bodies know that they do not allow conservative speech, or allow republican politicians from usage.
FreeRepublic is a conservative activist forum. There is no secret about it. The owner of this privately held forum is very clear about the mission of this website. He has made it clear that he will not allow this website to be used to promote liberalism.
And FB has made it clear via their terms of service that they can control what's posted on their site.
I may not like their decisions and if they piss off too many people their business will suffer, but as a private business they have the right to let anyone use their service or not provided they aren't discriminating against some protected class.