The cost of entering the mass-scale social media market is basically zero, since you can do that in a day with minimal cost other than a domain name and a server (or VPS or AWS/Azure account or similar), and you can scale your costs as you grow, but your costs don't grow as fast as your customer base.
A natural monopoly's costs start high, and just scales from there. It costs tens of millions for a utility just to enter a small market, and with utilities, growing may actually increase their costs (see, for example, power and water utilities).