WPP spent nearly $3bn on Google ads in 2014

"WPP, the world's largest advertising company, has just revealed Google is its biggest partner, in terms of where it plugs its clients' advertising money. And it's WPP's biggest media partner by quite some way.
Speaking on the company's fourth quarter earnings call, WPP chief executive Sir Martin Sorrell gave some interesting nuggets about the company's media outlay last year.
"Google has become our biggest media partner," he said. Last year WPP invested $2.9 billion of its $75 billion media bookings into Google ads."

Source:  Business Insider, 9th March 2015