Walmart and Amazon are both huge but it seems just wrong to say a brick-and-mortar store as equivalent to an online market place. The direct competitors to Amazon are Ebay, Baidu, Etsy, Temu and others, all of which are much smaller in the US (but some are huge elsewhere). You say that Facebook is a Google competitor because it offers eyeballs and because people sometimes go there to get stuff and it uses it's info for ads providing.

The actual situation imo is that online enterprises compete via monopolistic competition [1]. No large high tech company wants to offer exactly the same thing as it's because at best neither will make a lot of money - instead, any company entering a crowded marketplace will come up with something guaranteeing them more engagement, higher profits and so-forth (thus something even worse from the consumer's view). See Meta's Threads.

[1] https://en.wikipedia.org/wiki/Monopolistic_competition