Google and Facebook destroy competitors. And make sure new entrants cannot gain traction. They are about as anti-competitive as they can be.
I agree that they both hold onto their monopolies. But I don't think it's because they "destroy competition". Take Bing for example: it is quite alive. Google just delivers better search results. Yes the results have been deteriorating, but every time I get frustrated with them and try Bing as standard search engine, I switch back after a week. I don't like the google monopoly, but their core product is still not being challenging by anything I know of.
Their position as market share leader gives them everything they need to know to maintain quality. Any deterioration in search quality with the data Google has means far worse than the competition in relative terms.
Which is a complete shame since people like quality in absolute terms so it’s gonna stay that way. Even I’ve tried DuckDuckGo and Bing - regardless of how not-Google they are, the results are absolutely worse. So I’m not switching either, despite knowing how Google keeps its position.