Imagine the third largest country on the face of the planet, by population. Its most unusual characteristic: it's occupied entirely by Facebookians.

That would be Facebook's description, if it only had a land mass and a flag in addition to the billion users announced by the company on Thursday. In a blog posting, CEO Mark Zuckerberg said there are now "more than one billion people using Facebook actively each month." He added that "helping a billion people connect is amazing, humbling and by far the thing I am most proud of in my life."

One of Seven

Actually, the billion-user mark was reached on Sept. 14, meaning that one of every seven humans knows what Like buttons and personal Timelines are.

But, while 1 billion users is cool, 2 billion is really cool -- to paraphrase a line of dialog in the movie about Facebook's creation, The Social Network. Zuckerberg told Bloomberg Businessweek that he's holding a "hackathon" for his company's employees, to think about "ideas and working on prototypes and things that we'll need to do to help connect the next billion people." The jump from 500 million to 1 billion took a bit over two years.

As impressive as a billion users are, especially since the company only started in 2004 in Zuckerberg's Harvard dorm room, they are even more impressive when they're not considered as separate data points. Each user brings their circle of friends, their interests, and their history, and these social graphs become exponentially stronger with each new user.

In fact, those 1 billion users have resulted in 140 billion friend connections on the social network.

'No Real Difference'

Zuckerberg has been clear that he sees the immense, combined scaffolding of this social graph to be the real potential game changer as Facebook grows from huge to immense. He's said that the social network could become part of financial and health networks, for instance, as companies look to create services that "assume they can get access to knowing who everyone's friends are."

One key question is if there is a ceiling to how many users Facebook can acquire.

Brad Shimmin, an analyst with industry research firm Current Analysis, said the extent of the social network's growth would be "limited by Internet-capable devices," adding that one of Facebook's unique characteristics is that it is popular in many countries, while other social networks have not extended beyond their specific country or region.

About 2.5 billion of the seven billion humans on Earth have an Internet connection, and Facebook says it currently has about 600 million mobile users.

Ross Rubin, principal analyst at Reticle Research, said that, at the moment, there's "no real difference in the business impact" between, say, 900 million users and a billion. But, he noted, device makers such as Nokia have been populating the developing countries with handsets that can access the Net -- so there's still a lot of headroom left.