Earlier quoted context omitted.
If you're 15, you're not allowed in a bar, and you're not allowed on a porn site, and you're not allowed to buy cigarettes. The list goes on and on. I fail to see how not being allowed on Facebook is different.
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).
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.