The project will focus on expanding wireless Internet access, a cheaper method for connecting, especially in remote areas, than laying landline broadband networks.
The companies will work together to develop low-cost smartphones and tools that reduce the amount of data that apps consume.
Google has its own project called "Loon" to provide Internet access in remote and rural areas using high-altitude balloons.
Connecting more people to the Internet can mean more revenue for Web companies like Facebook and Google. But Zuckerberg insisted that the project isn't about money.
"If we really just wanted to focus on making money, the first billion people who are already on Facebook have way more money than the next five or six billion people combined," he said. "It's not fair, but it's the way that it is. And we just believe that everyone deserves to be connected, and on the Internet, so we're putting a lot of energy towards this."