Sure, "Google" as a private entity can curtail freedom of speech on its platform. It becomes less clear when Facebook, Twitter, and Google are effectively public goods.
Except they're pretty clearly private companies, not Public Goods. And as such, they can do basically whatever they want about speech on their platforms.
At which point does a service offered by a private company become a public good? There was such a transitioning point for electric power, for telephone service, etc. Will it occur for social media? And if so, at which point?