FB and FR both have terms of service. FB’s are less restrictive than Jim’s but explicit when you sign up for an account.
On both services anyone can sign up and post until the owner decides that they can’t. For any reason.
There is no difference in a legal sense.
Free Republic is an explicitly conservative, right-wing website.
Facebook is not.
That is the difference the courts will look at. And in any case, we can write in the law, "we exempt political/social forums and refer to only social media websites like Facebook." Easy Peazy.