[shadowgovt]

You've hit the nail on the head regarding the difference between the EU concept of antitrust and the American one.

EU antitrust law is grounded in harm to companies. If there are many companies, but one has sewn up the market so strongly that others can't really have a chance to prosper, the EU will step in.

American antitrust law is grounded in harm to consumers. The US regulators won't go after a successful ad company because other ad companies say it's too hard to compete with them; it'll go after them if consumers say "We don't have any choice but to do business with X, and X's prices are too high / X is too exclusive / X is only doing business with us in market sector A if we agree to use them also for market sector B."

That additional bar to regulation means the EU may very well have cause to act before the US does if Google is big enough to muscle around other ad agencies and set the terms by which they can exist, even if they do exist.