Martin Sorrell at the World Economic Forum in Davos, Switzerland on Jan. 17, 2017.

WPP clients invested $90 million in Snapchat ads last year, but spending on the social media upstart was overshadowed by far more established networks Facebook and Alphabet's Google, Sir Martin Sorrell told CNBC on Friday.

Sorrell spoke a day after Snapchat's parent, Snap, officially filed for its hotly anticipated initial public offering.

"Google, we invested on behalf of the clients about $5 billion, a little bit under," the WPP CEO said. "Facebook, about 1.7 [billion]. So there's a lot of room for Snap to develop or Snapchat to develop beyond that."

But there's no denying Snap's ad sales are catching up with its competitors', Sorrell said.

"I can see Snap revenue is about $400 million," Sorrell said, noting that WPP's clients dished out nearly a quarter of the social media company's net revenue for 2016.

In its filing, Snap said its net revenue grew to $404.48 million in 2016 from $58.66 million in 2015 but also said it "may never achieve or maintain profitability."

CORRECTION: This story was revised to correct that the $90 million spent by WPP's clients represents nearly a quarter of Snap's 2016 revenue, not of the amount Snap spent on ads.