I don't think Free Speech has ever meant being required to publish others speech. Freedom of the press was meant to ensure you can say what you wish via pamphlets or starting your own newspaper, wherein you own the method of speech. That is why it was fundamental to Free Speech.

One has never been able to compel a publisher to publish them.

You may have a different concept, but to my way way of thinking that has always been the connection between the First Amendment and Free Speech.

Freedom of speech is really something that must be contextualized, for it to be a useful term.

In all contexts, freedom of speech is considered a 'right', but in some contexts it's a positive right, and in others it's a negative right.

In the US constitution, it's a negative right. The First Amendment states that 'Congress shall write no law ... abridging the freedom of speech.' In the Universal Declaration of Human Rights, it's a positive right:

    "everyone shall have the right to freedom of expression; this right shall include freedom to seek, receive and impart information and ideas of all kinds, regardless of frontiers, either orally, in writing or in print, in the form of art, or through any other media of his choice". [1]

If Twitter were subject to the UDHR, it would seem to me that people would be entitled to use Twitter as a free speech platform. We are lucky that this is not the case, because that would surely violate Twitter's property rights and its employees'/investors' right to self determination.

To dissallow Twitter to shape its own content/user base strategy would be unethical.

[1] http://www.claiminghumanrights.org/udhr_article_19.html

The concept of Free Speech also includes that of Free Association. As in, Twitter gets to decide who they want to associate with. And in this context, associate with means use their platform.