And if you wanted to place an ad online to say, reach everyone in the US of driving age (car ad), or in Toronto during an NBA game (Pizza ad) what are all your options?
If you wanted to place a television ad to reach everyone in Toronto watching an NBA game, what are your options? You buy from the people who literally negotiated an exclusive right to air said game.
Now you're talking about the ad business, not the search business. In truth, Google isn't a search company, it's a advertising company with a search engine for market research. Once you look at it that way, you realize that Facebook and Amazon (to a certain extent) are direct competitors to Google, and they can't all be monopolies at the same time.
> "...and they can't all be monopolies at the same time."
semantically, that's true (since mono- implies one) but that doesn't mean that they can't all exert monopoly power in the same market and be an oligopoly. each establishes unfair advantages in the same market and stifles competition, like effectively excluding new extrants, for instance.
monopolized markets aren't simply defined by the number of dominant players, but rather by how fair and competitive the markets are. while market share is an often-present characteristic of monopolists, you could have one player with 60% market share that doesn't have sufficient market poewr to raise prices. and higher profit margins resulting from undue influence over a market (with or without significant market share) can be the telling characteristic of a monopolist.
in constrast, (asian) night markets seem to be a paragon of competitive markets, where there will be 29 competitors for the same product all lined up in a row.