Obviously, a website with a few hundred or few thousand regular users is more like a private club. A website with tens or hundreds of millions of users is more like the telephone company.

There is no clearcut boundary, but there doesn't need to be. Competition regulators frequently impose limits on market-dominant firms which they don't impose on small players – yet there is no clearcut boundary between a market-dominant firm and a small player. In practice, many individual cases will be obvious, and in the non-obvious cases, all we need is someone with the authority to make a decision–and if someone else thinks they've made the wrong call, there are the usual judicial and political processes to address that.