For one thing, Internet advertising is still not a majority of all ad spending. Google is estimated to control about 1/3rd of the online market, Facebook about 1/4.
This reminds me of a Peter Thiel saying. Something like, when you're not a monopoly you lie to pretend to be one - "We're the only British breakfast restaurant in town" to attract investors. When you are a monopoly you lie to pretend not to be one by expanding your market - "We're a technology company".
Internet advertising may not be a majority of all ad spending, but Google is an internet ad company, not a general ad company. It would be like arguing the old phone company wasn't a monopoly because plenty of communication was still by mail or in person.
