Amazon is most like Facebook in that the ads are only displayed on its own property rather than all across the whole internet on any site that displays 3rd party ads. That, combined with the fact that Amazon also has a ton of data on its users just like Facebook makes them very alike (not that Google doesn't as well).
What separates Google from Amazon is that their paid ads can be displayed on any 3rd party site that they don't own, through DoubleClick. And of course the whole search engine monopoly makes that impenetrable for now.
Of course, it's all up for grabs, Amazon could go out and buy an ad network like Mediavine and go head-to-head with DoubleClick.
We've been displaying Amazon CPM display ads on a site for over 2 years now. This is an ad network for affiliates, but paid by CPM. It's mostly a mix of large brand adverts and Amazon remarketing ads (normally for products you've looked at).
The service is about to be shutdown though at the end of this month, and their suggestion is that we apply to their larger header bidding network (Unified Ad Marketplace), but not sure we'll qualify for that.
Facebook has a display network similar to DoubleClick, called Audience Network. In my experience it pays publishers a lot better than DoubleClick. For advertisers, they're able to target on more demographic categories (for now, the CA scandal has done a lot to reduce targeting options).
A significant chunk of that ~4 Billion or ~5% of total global online advertising is because Google also owns YouTube, which captures a large chunk total advertising spending independent of search.
Twitter also has an ad product called "Twitter Display" in which your promoted tweets get reformatted into display ads and shown in other ad-supported mobile apps. It was some of the cheapest, but worst-performing, traffic we've ever driven through a paid channel.
I've seen lots of money wasted on blogspam websites within GDN (non-retargeting) -- there's a need to be very hands on if you don't want to waste money across all display tactics, in my opinion
More importantly, almost all of Amazons searches are commercial in nature, whereas I think for Google it is only about 30%, the other categories being navigational and educational. This means that their data is much more monetisable.
What separates Google from Amazon is that their paid ads can be displayed on any 3rd party site that they don't own, through DoubleClick. And of course the whole search engine monopoly makes that impenetrable for now.
Of course, it's all up for grabs, Amazon could go out and buy an ad network like Mediavine and go head-to-head with DoubleClick.