> I'm pretty confident that if a suitable case made it to the supreme court, social media like Twitter would be deemed a de facto public square under the US Constitution.
1) Everyone agreed that there was no chance of the presence of petitioners implied that they were endorsed by the shopping center. This is key: Pruneyard's speech isn't being impacted, so such a law isn't infringing Pruneyard's rights. This does NOT hold true for social media--the reputation of even the largest social media sites is driven by the content posted on them, which means any restrictions on sites' ability to regulate their own posted content necessarily infringes those sites' rights.
2) The California state constitution has a stronger guarantee than the US constitution, and the right of the protestors to set up a table was only granted by the former and not the latter. But this is only permissible so long as it doesn't infringe the US constitutional rights, which, see the first point.
The issue I see is: that was an outdoor shopping mall where you didn't have to check an "I agree to the terms and conditions" in order to enter. And Twitter's T&C say they can suspend you for any reason.
My point is that most case law was ruled on when businesses were an order of magnitude smaller, and had smaller impact on the individual.
Sometimes difference in scale is also difference in kind.
A lot of prior case law that involved much smaller private entities would be unlikely to hold if retested at current scale, against e.g. Facebook or Twitter.
Even if the mechanism isn't through the constitution, Congress is likely to enact similar protections through new laws sooner or later.
Any case law to that effect?