I think the argument is that there are no constitutional rights to bars, cigarettes, and porn. Social media is being held up as an example of "speech" here, which all US citizens are entitled to (at least in theory).

I’m not sure I buy “social media” = “speech”. By that definition, social media companies shouldn’t be able to prevent anyone from using their website for any reason. Being banned for anything would then be equivalent to violating someone’s rights to speech.

Social media companies aren’t required to give a voice to anyone. They are platforms for enabling exercising speech, but they aren’t speech themselves. This seems equivalent to stating that not allowing children in bars or strip clubs violates their right to assembly.

Social media isn't controlled by the government though.

The question is whether the venue in which the speech is performed in is sufficiently public as to be exempt from regulation. Given that participation on these sites exposes participants to heavy commercial advertising, traffic analysis, and data harvesting, I think it's a reasonable stance that the state can regulate participation.

Social media is not “speech” it is a specific type of platform which you also do not have a constitutional right to.