The jurisprudence, largely from "conservative" judges, is that "speech" is any expression, and it applies to corporations. Ironically, that's what'll make it tough to regulate Facebook/Twitter/etc. They didn't see the 2nd order affects coming.

Here's the 1st amendment:

"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."

What works for me is to replace the words Congress & government with Facebook, Twitter, or any company with a certain market share. If you want to be a corporation that facilitates communication between thousands of people, you're subject to the rule, ie, you lose some of your private property rights. If you have fewer than say 1000 users, do what you want: only private property rules apply.