Three ways that I can think of quickly:

1. Ads cost more which raises the cost of doing business which raises the cost of goods.

2. It also makes certain borderline businesses infeasible -- there would be a whole class of businesses that don't/can't exist because they become unprofitable if you have to pay the ad monopoly/duopoly piper on top of normal expenses.

3. You could also argue that FB's continued dominance is another societal harm stemming directly from the alleged actions. If there wasn't an ad cartel, it would be easier to start new ad-based social media companies. (Wouldn't it be crazy if Google+ being neglected and then shutdown was a part of the negotiations between Google and FB?)

Lots of presumptions in the above, but I think a strong case could be made for each.

Because more competition in the ad space is invariably better for consumers. The alleged practices at play are illegal because they are known to harm consumers. And this is sufficient to demonstrate harm. For example, collusion is illegal not because someone necessarily sues and says “I was harmed by this” but because collusion invariably harms consumers. That’s the whole motivation behind collusion/price setting. Seems clear as day to me though I inquired with my friend who works in antitrust litigation and this was the response:

“There’s the broader principal that if they’re setting prices illegally they may not be harming consumers right now but they certainly could. Second, consumers are hurt when innovation/competition is curbed, and that is what they allege the deal’s purpose is. The bottom line is that if there were illegal deals and they can prove there were, those agreements are per se violations and you’re not really looking at consumer harm. This is what the case is alleging.”

The brief actually sums this up really well. The claims start at p. 100, which is where they allege harms but they provide good explanations throughout.

Edit: Note about the the brief.

> Can anyone quantify the harm to the public?

Drive down ad rates for non-google content, which severely hurt other digital publishers, like news sites. This made it basically unsustainable for them to survive on advertising. Thus, hurt local reporting and investigation, which in turn allows corruption and ineffective government, eventually hurting democracy.

The business has to pay more to acquire customers, which drives up the cost of goods. Indirectly the consumer pays in mobile bandwidth costs, and publishers are forced to show ever-more ad units, refresh every 30 seconds, paginate articles, or add content recommendations, all because AdTech takes 85% of the ad spend that use to flow more directly between publishers and advertisers. Much of that went to the cost of staffing a sales team, but those folks were doing human review on campaigns, so ad quality was high and fraud was low. With unlimited digital ad variations, ad networks have to police a mountain of potential bad ads, which can be directly malicious to the user.

I can spin up 1000 banners and text ads, but only spend a dollar. See the problem? They all still need review. Instead, GOOG relies upon AI and user reports, which they promptly ignore. Meanwhile, they serve up so many mobile redirects that publishers have to pay yet another vendor for protection against the crap Google doesn't vet. As a publisher I'm not really interested in showing my audience ads that automatically redirect them away from my content and to a scam $500 Walmart Gift Card landing page. Yet, if I want to have access to Google demand, I have to take the bad with the good. What choice is there? Exist and use Google, or pass on 60-90% of revenue while competing for the same audience?

As an advertiser I have to count on Google's reporting to tell me if I'm making positive ROI and they are constantly finding new ways to convince me that they made the sale. They bill for ads that were never seen and clicks that went to nowhere. Occasionally you'll see a discount on your bill for "Invalid Clicks" but you'll never see anything auditable about it. If I don't police my site list, I might end up funding websites run by terrorists and/or money launderers. Honestly how many US citizens who are shopping for cars do so on .ru domains? If you're not vigilant, that will be 20%+ of your ad spend. Yet, if not Google or Facebook, where can I reach a relevant, large audience with a small and variable ad spend? Not with DSPs, not with agencies, not with direct ad buys.

One has to have a theory of (public) harm. There are a bunch of them from a legal perspective, and maybe they will carry the day. They don't necessarily make sense from a non-legal colloquial perspective.

From a non-legal perspective, one of the big harms (I would argue) is in the destruction of the business model for small ad-supported publishers. Google was taking 80% of money that should have gone to them, and digital and physical publishers (like local newspapers) have had a mass extinction.

That said, out of the ashes, rises the phoenix. Substack and other subscription models owe their opportunity to Google's monopoly, and in many ways they're the mammals in this ecosystem of dying dinosaurs.

This affects the whole online ad market, not just Google's competitors. That's a whole lot of "the public" that's directly affected.

How do you quantify it? I don't know, how about we look at how much money Google is making?