What's the difference between this and Google's "monopoly" on search, which is again based on a superior product (You could argue the same about Facebook, but at least in the search case, there's no network effects tying people to using Google).
FB has a monopolistic-power in the network effects like you said, but even then entrants like Snapchat show that the network effects can be gained for new competitors.
Now Google may not have done any of the above with search, but it's likely possible to argue that they have unfairly promoted their own products over others, or "unfairly" chose to not list certain websites.