it's the other way around. Facebook has always been a private concern. At some point it became an effective monopoly. That doesn't stop it from continuing to be a private concern. The process of going from private concern to public concern is called "nationalisation"; the government forcibly buys out the shareholders of Facebook and runs it as a public asset.

The shareholders of Facebook are entitled to run their company as they see fit. Because they own it. Like you can paint the walls of your house whatever colour you like, and if visitors don't like it, then that's their problem.

If we think that Facebook has grown too powerful and intrusive into society to be governed by its shareholders, then nationalisation is completely the correct way to go.

Of course, which nation gets to buy Facebook is another concern... given its global reach, is it really appropriate for the US government to own it? Should the UN "globalise" it instead of the US "nationalising" it?