Four years ago, the United Nations predicted that more than half of the global population would be connected to the internet by 2017, buoyed in part by “the fastest growing technology in human history”: mobile broadband. The world missed the mark. Now the UN expects to achieve that goal by the end of 2019, and that still leaves an estimated 3.8 billion people offline.

What happened? Though global access to the internet grew between 2002 and 2016, the rate of growth has slowed in the past two years, according to an analysis from the Web Foundation that was first reported by The Guardian.

It's important to be clear that slower growth doesn't mean people aren't still gaining access. You could have the same number of people get connected every year, and the percentage growth rate would drop. What surprised the Web Foundation team, led by research director Dhanaraj Thakur, was how much the gains have slowed: The connected population grew by 19 percent in 2007; last year it grew by less than 6 percent. Thakur is still studying exactly what accounts for the slowdown.

"If the growth rate is slowing down, that’s very worrying, because when will that next 4 billion people get online? It’s going to take longer for that to happen, and that’s a big problem," he says, pointing to the economic and social opportunities that are cut off from those without access to the internet.

But the challenges of universal access, or even reaching the UN’s goal of 50 percent, go beyond the recent slowdown in growth rate. Hidden in the broad number of people the UN already considers to be online is a bleaker picture, one in which even that access is severely limited by factors both economic and cultural.

The International Telecommunications Union, the UN agency tasked with getting the world connected, counts a person as using the internet if they managed to get online at least once in the last three months. That, some advocates say, shouldn’t count as meaningful access. “The economic benefits of having internet access don’t kick in or become real until you are using the internet consistently,” says Ellery Biddle, director at Global Voices Advox, an online network dedicated to free speech and internet access issues. “I don’t think using it once every three months would have much of a benefit.”

The UN’s measurement has other weaknesses, too. “When you talk about getting online, it’s not that simple. Not everybody is experiencing the internet in the same way, and that broad metric hides that fact,” Thakur says. “In some countries, there is very fast connectivity. In others you might have a spotty 3G connection that goes in and out, so what you can do with it is really restricted. This definition does not capture that.”

“It’s not geography, it’s not mountain ranges, it’s lack of political power in a lot of cases that’s determining access.” Peter Micek, Access Now

And while the UN has touted mobile broadband as a key instrument for achieving universal access, that technology remains out of reach for many around the world due to cost. In a survey of 60 low- and middle-income countries, the Web Foundation found that only 24 offered affordable mobile plans (costing less than 2 percent of the national average monthly income) at the end of last year.

Even when infrastructure is in place, high rates can persist if telecommunications companies have a monopoly, or if they depend on foreign companies to manage their networks, which costs a lot of money. Some countries have made good use of Universal Service Funds, which are created by charging fees to telcos, to subsidize lower-income access to the internet, but others leave the money on the table.

“A lot of the easy work has happened, in terms of wiring up cities and getting developed nations online. I think we’re getting to those obstacles that are very much tied in with a lot of the other economic and cultural challenges that we face,” says Peter Micek, lead counsel for the advocacy group Access Now and co-chair of the Gender Digital Divide working group at the IEEE's Internet Initiative. “It’s not geography, it’s not mountain ranges, it’s lack of political power in a lot of cases that’s determining access.”