--Tears for Fears
Donald Trump is suing several social media companies over their efforts to censor him. Critics argue that the suit can't possibly win because companies like Twitter and Facebook are privately owned.
The issue is not ownership, however. The issue is whether an entity is open to government influence. If, for example, Twitter gets resource grants from the government, or if it receives favorable regulatory treatment, then the company is susceptible to influence and a potential tool for government-sponsored action.
A plaintiff should not even have to prove that Twitter has acted on behalf of government. It merely needs to demonstrate that the company has received favorable government treatment and is therefore 'available' to the government.
If Twitter has received favorable government treatment, and there can be little doubt that is has, then it is essentially a 'government sponsored corporation.' A GSC must abide by the same rules that apply to any government agency--including those that govern free speech as specified by the First Amendment.