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These success stories don’t prove that it’s feasible to create a widely used social network, just that it’s possible. And that’s a pretty obvious statement, seeing that widely used social networks exist at all. What’s important is how many failures are there?

There’s no physical issue preventing competition like with power companies, but to a social media startup, getting enough users is a similarly hard challenge as constructing a new power grid.

And you’re not actually a social network until you reach some critical mass of users. The cost of entering the market is low as you point out, but that doesn’t imply that you can realistically compete. So I think it’s fair to call social networks a natural monopoly. Even if they don’t strictly fit the definition they’re still monopolies in practice.