(1) There is a better solution. The problem is that they are monopolies. So bust the monopolies. Require interoperability — a proven solution in other industries. A service needs to be treated as a utility — meaning large amounts of regulation — when it is a natural monopoly, meaning there’s no practical way to avoid a monopoly. That’s not the case here. Don’t use a ton of regulation where a much smaller amount will do.
(2) It doesn’t solve the problem. A social network utility still needs moderation. How would making a social network a utility make the moderation decisions better than what you’re getting now? Complicating things, in the US the first amendment means the government can’t establish laws to limit free speech, so a public utility would have to be a minimally moderated cesspool.