This FT article is pretty interesting:

The classic example of industrial-era price fixing dates back to a series of dinners hosted amid the 1907 financial panic by Elbert Gary, then chairman of US Steel. In a narrow first-floor ballroom at New York’s Waldorf Astoria Hotel, men controlling 90 per cent of the nation’s steel output revealed to each other their respective wage rates, prices and “all information concerning their business”, one attendee recalled. Gary’s aim was to stabilise falling prices. The government later sued, saying that the dinner talks — the first of several over a four-year period — showed that US Steel was an illegal monopoly.

Algorithms render obsolete the need for such face-to-face plotting. Pricing tools scour the internet for competitors’ prices, prowl proprietary databases for relevant historical demand data, analyse digitised information and arrive at pricing solutions within milliseconds — far faster than any flesh-and-blood merchant could. That should, in theory, result in lower prices and wider consumer choice. Algorithms raise antitrust concerns only in certain circumstances, such as when they are designed explicitly to facilitate collusion or parallel pricing moves by competitors.

… a German software application that tracks petrol-pump prices. Preliminary results suggest that the app discourages price-cutting by retailers, keeping prices higher than they otherwise would have been. As the algorithm instantly detects a petrol station price cut, allowing competitors to match the new price before consumers can shift to the discounter, there is no incentive for any vendor to cut in the first place.

“Algorithms are sharing information so quickly that consumers are not aware of the competition,” says Mr Stucke. “Two gas stations that are across the street from each other are already familiar with this.” This episode suggests that the availability of perfect information, a hallmark of free market theory, might harm rather than empower consumers. If the concern is borne out, a central assumption of the digital economy — that technology lowers prices and expands choices — could be upended.

The argument here, if it is right, is twofold. One – that even without direct collusion, firms’ best strategy may be to act as if they are colluding by maintaining higher prices. Firms have a much weaker temptation to ‘defect’ from an entirely implicit bargain by lowering their prices so as to attract more customers, since there are unlikely to be significant gains from so doing, even in the short run. The plausible equilibrium is something that might be described as distributed oligopoly. Harrison White once defined a market as being a “tangible clique of producing firms, observing each other in the context of an aggregate set of buyers.” With super-cheap information, it doesn’t have to be a clique any more to be tangible.

The second is that where there is direct collusion, the information burden on regulators is much higher. For example, one may plausibly imagine that oligopoly-type outcomes might emerge as a second-order outcome of the aggregated behavior of automated agents. One might also imagine that it might be possible artfully to tweak these agents’ behavior in such a way that this will indeed be the most likely result. However, proving ex post that this was indeed the intent will likely at best require a ton of forensic resources, and at worst may be effectively impossible.

NB that both of these can happen entirely independently of traditional arguments about concentration and monopoly/oligopoly – even if Amazon, Google, Facebook, Uber etc suddenly and miraculously disappeared, these kinds of distributed or occulted oligopoly problems would be untouched. If you take this set of claims seriously (the evidence presented in the FT piece still looks tentative tentative), then the most fundamental problem that the Internet poses is not one of network advantage, increasing returns to scale and so on advantaging big players (since, with a non-supine anti-trust authority, these could in principle be addressed). It’s the problem of how radically cheaper communication makes new forms of implicit and explicit collusion possible at scale, squeezing consumers.