Google controls the vast majority of the web search market. More than 90%. What exactly a monopoly constitutes is something that people can disagree about, but "has 90% market share" is not by any means a crazy definition of a monopoly.
Google ad revenues mostly come from 3 services: Gmail, which holds a disproportionate share of all email for what started out as a federated network. YouTube, which basically holds a monopoly on video sharing. And Google search, which basically holds a monopoly on regular web searches.
I count at least 2 monopolies here, both held by Alphabet. The fact that Facebook is able to make advertisement in some other part of the web is immaterial, the same way TV ads are immaterial.
Facebook has 90%+ market share of social media. Do they have a monopoly?
GitHub has a 90%+ market share of open source code hosting. Do they have a monopoly?
I don’t know whether this kind of market dominance factors into the legal determination of monopoly, but conceptually I think it makes sense to say that Google has a monopoly in the web search market.
The customer in a web search isn't the USER. It's the BUISINESSES whose ads are placed on the search results page.
THEY are certainly paying for the web search.