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.
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?