[curryst]

> There are plenty of options out there, and the capital required to start one is pretty low compared to something like a internet service provider or a train network.

A social media network with no users costs about as much as a single train station that doesn't connect anywhere: $0. They're also about equally useful.

A social media network with enough users that people can talk to their friends on it is enormously expensive. It takes a lot of advertising and pushing to build up that base. Probably not as much as a train or ISP network, but certainly far beyond trivial quantities of cash. Google+ was estimated to cost $585m, it was from a company with a major internet presence, and it still failed horribly.

You're also not considering that network effects heavily encourage a monopoly. If your cell phone could only call people with the same provider, you don't really have any meaningful choice. You pick whichever provider the people you want to talk to are on. In cases where consumers don't have meaningful choice, it makes sense to add additional protections for consumers.