Only 40% of the world has ever connected to the Internet and the unconnected mostly live in developing nations, according to a new study published Monday by Facebook-led Internet.org.

The study, which outlines the state of global Internet connectivity, also found that 37.9% of Earth's population uses the Internet at least once a year, but more than 90% of the world's population, at least, lives within the range of a mobile network.

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Moreover, the majority of people in developed countries are online—78%—while just 32% in emerging economies use the Internet.

The disparity has been highlighted before: People in developed countries have a reasonable expectation of Internet access, while those in developing economies are more likely to be left out.

Image: Facebook

This year, Internet.org expects 3 billion people to have Internet access. But adoption is slowing — for the fourth consecutive year. Internet adoption grew by just 6.6% in 2014, as opposed to 14.7% in 2010. At this rate, Internet users won't reach 4 billion people before 2019.

Still, there is some promise. Nearly 80% of the world's population, according to the study, can afford the Internet.

"This means that we will need to look at issues like affordability and awareness in order to connect the majority of people," reads a Facebook blog post about the study.

Image: Facebook

Facebook identifies the three barriers to access as infrastructure, affordability and relevance. The relevance part refers to people who are "either unaware of the Internet" or it doesn't have enough content in their primary language.

You can read the full report here.

Other organizations have studied global Internet access, such as the United Nations. Its 2014 report mostly mirrored Facebook's, but also projected that two-thirds of people in the Americas would be using the Internet by the end of that year.