On Tuesday, Facebook announced that it had hit a huge milestone: 2 billion people now use the platform each month. That makes Facebook larger than the world’s most populous country, China (1.37 billion), the fortune of Chase Coleman III (1.9 billion), and the number of miles between the sun and Uranus (1.78 billion). How long will it take the site to add another billion users?

Facebook was first opened to the public — that is, anyone over 13 years old with an email address — in September of 2006. Six years later, in October 2012, the platform reached one billion users; and now, less than five years later, Facebook’s already doubled in size. It could reach 3 billion users by the middle of 2021 if it keeps growing at this rate. But that’s unlikely.

One of the largest hurdles the company faces as it attempts to expand its reach even further is that most of the global population still doesn’t use the internet. Only about 47 percent of the world is online, and many of those with internet access have it only sporadically.

China and Iran have banned Facebook: Together, these countries make up about 1.5 billion people. Mark Zuckerberg is hard at work trying to convince China to return to the platform (it’s been censored there since 2009).

Some reports say that kids are ignoring Facebook — calling it “meaningless” — and instead opting for other social media platforms, like Instagram or Snapchat. At the same time, older folks are enthusiastic about Facebook.

In many countries — especially the United States — it’s becoming harder to get by without a Facebook account. Many businesses use a Facebook page instead of a website. Apps and services sometimes require a Facebook account, like the ever-popular Tinder. And every day the site becomes a hub for more groups and organizations.

It’s hard to say where Facebook will be in another five years, or whether the site will ever reach the 3 billion mark. To pull it off, Zuckerberg will have to woo some foreign governments, expand into places that have only recently gotten online, and manage to keep his platform relevant.

Photos via Getty Images / Paul Marotta