I don't think that's really fair. By that argument, every magazine, radio, and TV show are ad companies. That includes NPR which runs ads for sponsors and ads for pledges. NYTimes, The Guardian, etc...
Do all those companies make money from ads? Sure. Does that make them ad companies? No, they are first and foremost companies that provide a service for the user. If that service is good enough (gets enough viewers) then the ads are seen/heard. If not they go out of business. Because of this they have an incentive to provide the best of that service. Best search, best TV shows (drama/news/comedy/etc...), best music, best talk shows, best articles, etc. Where "best" = gets the most viewers. If Google's search starts sucking people will go elsewhere so they are a search company. Chiat Day and Dentsu are ad companies. They sell ads.