Notice how I didn't mention social media should be a monopoly but a public good. While I do believe electric power production, water extraction, parcel distribution, telephone and Internet access should be public monopolies (again for most of them), social media should just be a public good.

Contrary to what you said there is little money in social media itself, all revenue Facebook and Twitter are getting comes from advertisement and mass surveillance, not from individuals paying for social media like we are paying for electricity, water, telephone or internet access.

With social media as a public good, a state could forbid proprietary social media protocols, provide their own instances for open federated ones and still allow individuals to run/make their own platform.