And while Google has monopsonic power, it isn't the sole player in the market so it's not a true monopsony. Facebook, Twitter, and other channels have control as well. If you still disagree, look at how widespread Facebook's open graph standards are as proof of the market catering to these companies' purchasing power.
Google isn't a monopoly for users, it's a monopoly for developers. You can't get users without being on Google.
Only if I can replace one company with another, at roughly equal results, can the market be considered competitive.
I can't delist my site on Google and replace those users with Bing users.
If each search engine had only 5% or 10% of the market, though, I could. And I might. That's what a competitive market looks like.
Google is, from that perspective, a monopoly.