Once a company becomes as big as the likes of Facebook, Google etc they start to talk about monopolies and breaking them up. But in the tech industry things can change so quick that someone who once dominated can become an also ran almost overnight as customers start to flock to new startups offering something fresh.
Look at the likes of Myspace how quickly that was abandoned when Facebook came along. And Facebook would today be a lot more irrelevant had it not purchased Instagram which is used by a lot of young people who prefer it over Facebook. Sega and Nintendo ruled the consoles arena in the 8 and 16bit era were replaced by Sony and Microsoft once the move to 32bit started.
I am not saying that something shouldn't be done to about the power these companies have, but I think blocking acquisitions of smaller competitors would be a better start. Something if they had done back in the 1990s when Microsoft were buying anyone who made a half decent competitor product, might mean we had less vendor lock in, in the desktop OS and Office software today.