I don’t know for a fact that the Amazon.com bookstore is the world’s biggest OLTP application — but if it isn’t, it’s close.
And the thing is — that’s never been an entirely relational application. Oh, the ordering part surely is. But the inventory lookup is currently driven by an OODBMS (from Progress). The personalization used to be done in Red Brick (I knew which software replaced it, but I’m forgetting at the moment — it may even be one of the relational warehouse appliance vendors). And of course the full-text search is a custom in-house system.