Look at FB stock, it just dropped from 220 -> ~130, they saw almost half of their market cap wiped away in a few weeks. The monopoly idea is a narrative invented to stoke fear, outrage, and page clicks. It's a talking point for psuedo educated people.
I also don't think bringing up FB's stock price is relevant, not sure what your point is getting at there?
If monopolies can routinely be outcompeted, then what is there to be concerned about with them?
Markets can change in ways that destroy monopolies. For example, if in 1900 you had a monopoly on horse-drawn carriages, that wouldn't have prevented you from being put out of business by the advent of the automobile.
I think the distinction is between a monopoly disappearing because the market it owns disappears, or because it gets outcompeted in it.
I'm real curious how you think those companies are irrelevant.