It's pretty clear (in the context of a monopoly complaint):
In March2017, Google’s largest Big Tech rival, Facebook, announced that it would throw its weight behind header bidding. Like Google, Facebook brought millions of advertisers on board to reach the users on its social network. In light of Facebook’s deep knowledge of its users, Facebook could use header bidding to operate an electronic marketplace for online ads in competition with Google. Facebook’s marketplace for online ads is known as “Facebook Audience Network” or FAN. Google understood the severity of the threat to its position if Facebook were to enter the market and support header bidding. To diffuse this threat, Google made overtures to Facebook. Internal Facebook communications reveal that [redacted]
and
In the end, Facebook curtailed its involvement with header bidding in return for Google giving Facebook information, speed, and other advantages in the auctions that Google runs for publishers’ mobile app advertising inventory each month in the United States. In these auctions, Facebook and Google compete head-to-head as bidders... [snip].. The parties agree on for how often Facebook would [redacted] publishers’ auctions—literally manipulating the auction with [redacted] for how often Facebook would bid and win.
> In almost all circumstances a market allocation agreement will be illegal under antitrust law. Federal antitrust law treats a market allocation amongst competitors as a per se violation of the antitrust laws. Other types of potentially anticompetitive behavior are only illegal if their anti-competitive effects outweigh their pro-competitive efficiencies. But per se violations are automatic violations of federal antitrust law.
This is 100% identical to the illegal stuff robber barons used to do
when two railroads were in competition and it was killing profits, they would sign a silent partnership and one would take upstate NY and the other would take NYC to other parts of NY
