If Google replaced all of their search results with ads, people would easily switch to bing as bing isn't that much worse.
Facebook on the other hand definitely is "pretty much" a monopoly (but not completely) for certain subsets of the social media market due to network effects which is why they shovel ads down your throat on their platform and they are making tons of money.
Their search results are good because they do machine learning on data from all their other users.
A lesser-used search engine has less data, so even if they have smarter people and a better algorithm, the search results will probably be inferior.
... and let me tell you Bing is not nearly as good as Google when it comes to bringing me the right search results first.
But I got tired of Google asking me to “prove that I’m not a robot” by tapping on roads and street signs with every new search. I use incognito mode, and since they can’t track me, they either are punishing me or just automatically assume I’m not human.