WhatsApp is well on its way to connecting 1 billion people, Zuckerberg said, and few services in the world attain that reach without becoming valuable. WhatsApp has more than 450 million users a month.
"I think by itself it's worth more than $19 billion," he said. "It's hard to exactly make that case today because they have so low revenue. … I could be wrong—there is some chance that this is the one service that gets to a billion people and ends up not being that valuable—[but] I don't think I am."
Earlier in the day at Mobile World Congress, WhatsApp CEO Jan Koum said it would be offering a voice service in the second quarter.
(Read more: WhatsApp will add voice to messaging service)
For Zuckerberg, WhatsApp coalesces with Facebook's aim of connecting everybody in the world, a goal that's also behind Internet.org.
Formed last summer, the initiative has Facebook, Ericsson, MediaTek, Nokia, Opera, Qualcomm and Samsung working with nongovernmental organizations, mobile operators and tech firms to make Web access more affordable and accessible.
Though many talk about 6 billion mobile users, what's forgotten is that many of those users do not have Internet access—not because smartphones are expensive but because data plans are, Zuckerberg said.
"Owning an iPhone for about two years costs about $2,000 in the U.S. About $500 of that is the phone and $1,500 is the data plan," he said, adding that some people aren't sure that they need such a service.
"You get caught in this catch-22," Zuckerberg said. "You have enough [money] that if you wanted to you could buy an Internet data plan, but you don't know what you would get with that because you've never had access with that."
Internet.org wants phone providers to offer basic services—including messaging, Facebook, weather, food prices and Wikipedia—free so that a wide population understands the Web's importance and signs on for more robust packages. That's when operators can make money from data, he said.
Zuckerberg emphasized that many apps must become more efficient, noting that the average Facebook user once wasted 14 MB of data a day on the app, whereas his team has cut that to 2 MB.
On another matter, the CEO said that while Edward Snowden's revelations about National Security Agency surveillance showed that the government "blew it" when it on transparency, the event had unified the tech industry.