The question I always ask my clients is, "What keeps you awake at night?" Almost without exception, the answer that comes back - whether I'm talking to a retailer or a brand owner - is "Amazon". Many of our own people at WPP say the same thing.
The list of insomniacs grows ever longer as Jeff Bezos's formidable creation relentlessly expands its territory and influence. Bezos is on a quest to make Amazon "the world's most consumer-centric company, where consumers can come to find anything they want to buy online" - a vision that places most businesses within Amazon's competitive set.
Two companies that don't lose much sleep over the competition, though (because, for the most part, there is none), are Google and Facebook. The duopoly's stranglehold on digital advertising means they will take as much as 75 per cent of global online ad revenues in 2017 and almost all new spending. But in 2018 they will start to feel the possibility of competition - and Amazon is one of the companies that might provide it.
Both Google and Facebook have grown fat by eating other people's lunch (that of traditional media companies in particular), but Amazon may be the one firm that is big enough to eat theirs.
Long the leading platform for product search (more than half of US online shoppers begin their searches on Amazon), it is now making a concerted effort to become a major platform for advertising too, with promoted listings, targeted ads based on users' purchase histories and a growing range of other options.
Amazon's market-leading AI and voice technology will inevitably form part of its offer to advertisers, and that's one of the reasons why the other four of tech's Fearsome Five (Google, Apple, Microsoft and Facebook), along with Alibaba and Tencent, are launching AI/voice technology too.
Read more: The Everywhere Store: Amazon's AI-powered master plan to be the world's biggest company
Amazon doesn't disclose its advertising revenues, but analysts suggest they were more than $1 billion (£780m) in 2016 and will be around $2.5 billion in 2017. Some estimates see them rising to nearly $6 billion in 2018.
While this is tiny compared to Google and Facebook - which between them had advertising revenues of more than $100 billion in 2016 - on a recent earnings call, Amazon said it was rapidly expanding its advertising sales team and was pleased with the results from this still-nascent business.
Advertisers want measurable return on their investment, and in this respect Amazon may hold the trump card. Unlike Google and Facebook, Amazon has a treasure trove of data on what consumers actually end up buying. Its 2017 acquisition of Whole Foods will add another huge tranche of data about shoppers' habits in bricks-and-mortar stores.
The more it figures out how to use those on- and offline insights to help advertisers, the more of a threat it becomes to the duopoly.
Amazon is not the only company vying to become the "third force" to challenge Google and Facebook in digital advertising. Snap may be struggling to meet Wall Street's sky-high expectations (expectations it did little to manage before its IPO) but it remains a contender. WPP's investment in Snap on behalf of our clients is growing rapidly, and will double this year to around $200 million. This, however, compares with almost $6 billion spent with Google and more than $2 billion with Facebook, so for now it's still just a flea on the elephant's backside.
Verizon's new AOL-Yahoo! combination, Oath, says it is not looking to take on the duopoly directly, but its CEO Tim Armstrong has pointed out that, with Google and Facebook, his organisation is one of "only three companies in the world that touch one billion consumers digitally". With an audience of that scale, and with targeted annual revenues of between $10 billion and $20 billion, there is little question that Oath also has the potential to broaden the choice for advertisers.
Elsewhere, telecoms giant AT&T plans to use its recent acquisition of Time Warner to build an automated ad platform for TV and premium video, while Pinterest has recently moved from the test phase of video advertising to a full rollout, hiring talent from Google and Facebook.
As much as advertisers would benefit from a loosening of the duopoly's grip on the market, it is unlikely that a true challenger will emerge as soon as next year. We should, though, have a better idea of who that third force might eventually be. (I'm making no predictions, and offer only this observation: few people have made money by betting against Amazon.)
And maybe the threat will come from elsewhere. In the longer term, the biggest threat to the duopoly (and indeed Amazon) may be success itself. As the dominance of these companies grows, they may increasingly be seen not as private corporations but universal public utilities - with all the regulatory scrutiny such a profound change in status entails.
This article was originally published by WIRED UK