I think it's a huge mistake to group Google, Facebook and Amazon all in the same bucket and say "look, they don't have a monopoly because they each only have X% of 'total digital ad spend'".
If I want to advertise my vacation house for rent, I sure as hell am not going to do it on Amazon, and FB would hardly make any sense (maybe if I had a whole network of houses for rent to do brand marketing, but not if I just had one). Google is pretty much the only option that makes sense here.
I mean, just ask a digital marketer. While there are some potential areas of overlap they all view Adwords, FB ads and Amazon ads very differently and not much in 'competition'.
Weird example because I am sure Facebook is a major outlet for vacation rental marketing. Here's the Vacation Rental Management Association's advice on using Facebook: https://www.vrma.org/p/bl/et/blogaid=1253
I imagine other large players in vacation rental marketing are VRBO, TripAdvisor, and AirBnB.
Anyway if you redefine the online advertising market to include only the things that Google dominates, such as search ads on Google itself, then sure you can make it look anyway you want.
I'm not "redefining" the online advertising market. If anything, practically every single digital marketer in existence considers, for example, FB ads to be very different from Google ads and serve very different purposes. It's you who have (oddly, IMO) redefined the the market by lumping everything together.
And look at your "other large players in the vacation rental market". Perhaps unsurprisingly some of those travel brands are among the largest spenders on AdWords. See Expedia (parent co of VRBO) Chairman Barry Diller's comments on this
exact topic: https://skift.com/2018/10/16/expedias-barry-diller-calls-on-...
If you look at search marketing, which is unique in its ability to target users at the moment of intent, let's look at the players. Google is at over 90% and everyone else is peanuts. I would agree with you that there is overlap with Amazon here, but only in a subset of products (i.e. primarily physical, deliverable products, not services).