I Facebook went out of business, people would instead message more on other messaging platforms, text people more, read news in other news platforms and so on.

I Facebook went out of business, people would instead message more on other messaging platforms, text people more, read news in other news platforms and so on. There are plenty of opportunities for all of those options. Consider, for example, the Cola soda market. There are countries in which Coca-Cola holds 95% or more of the market share for Colas, or even for sodas. Does that make it a monopoly? No. Because Colas are just one type of soda drinks, and soda drinks are just one type of drink. If Coca-Cola disappeared and there was no other Cola drink available, Cola drinkers would drink a lemon-flavored drink, or fruit juice, or just plain water. So your definition doesn't work, exactly because you didn't define the market. And hey, this isn't just my opinion, it is how monopoly is defined in economics. Particularly regarding something that we haven't talked about yet, which is monopoly pricing. There is no monopoly pricing in any market that Facebook operates. Quite the opposite, actually, ad prices have been going down continuously. It is always interesting to see people in HN trying to recreate economics from first principles.