That spending was "primarily related to the use of online search engines (primarily Google)," and other online services, according to Booking. Google is such a critical piece of Booking's advertising budget because so many travelers search for trips with terms like "flight to New York" or "hotel in London." Booking is competing for that highly valued traffic with Expedia, TripAdvisor, Trivago and even Google's own services. Mark Mahaney, an analyst at RBC Capital Markets, estimates that "north of 80%" of Booking's performance marketing expense goes to Google. Get the full story at CNBC