The social media giant bought messaging startup WhatsApp last week for a staggering $16 billion, plus another $3 billion in restricted stock options.
When asked on CNBC's SquawkBox whether the messaging service was worth up to $19 billion. Sorrell said: "Not to me, and probably not to you. But it is Mark Zuckerberg and Sheryl Sandberg and they are no dummies."
(Read more: Facebook buysWhatsApp: A desperate move?)
Facebook CEO Zuckerberg and COO Sandberg said WhatsApp would complement Facebook's existing chat and messaging services.
"On the surface it looks a little bit potty, but they are not potty and they know what they are doing. They have built a company in a very small period of time into a giant and I am pretty confident it will continue," Sorrell said.
WPP increased its share buyback program after reporting record profit margins and strong results in 2013.