I agree that they will rule with the large corps. But there sure is legal precedent for making them respect constitutional rights for everyone.
Bock v Westminster Mall: because the mall owner allowed a wide range of activities such as Christmas events, pageants, military recruiters, commmunity events, etc inside the common area of a mall, the Colorado Supreme Court ruled that the mall was a “latter day public forum” and could not eject protesters, and had to respect their 1st amendment rights.
Marsh v Alabama: the US Supreme Court ruled that a private company town had to permit free speech activities because it acted like a de facto real town.
The common element seems to be:
Allow public to have access, allow a wide range of activities other than the original narrow scope and allow government offices to set up.
The reason I believe FB, YT, Twitter and Google meets this requirement is because anyone can see a Channel, page or feed unless it is marked private, they all allow cities and recruiters to “set up shop” and they allow a very wide variety of activities. For example on FB you can post pet adoptions, shop (marketplace), have a business or personal page, go sign up to vote or contact your favorite politician.
If FB was just for talking to your relatives then it would never meet the requirements, but for the benefit of the stockholders, they promote a large variety of activity. Read the Bock v Westminster Mall rulings and it seems like it fits exactly what FB, Twitter and YouTube do.
I hope for all our sakes that you are right.