The company's hypergrowth began long after younger Americans had adopted the service.
If you were in college in the mid 2000s, you think you saw Facebook's fastest growth period. That's when the site was being deployed school-by-school across the country. In every individual school, Facebook's growth followed pretty much the same exponential curve from no one to everyone in a matter of weeks. That is to say, for me personally, Facebook's main growth story was over by September of 2004.
Later, as the company pushed outside colleges to high schools and the military and then to everyone in September 2006, individuals' social networks tended to follow a similar trajectory. Once a few people were infected by Facebook in your circle, soon everyone else was, too. For the average reader of this blog, a tech-savvy someone in his or her late 30s, Facebook's conquest of our social networks happened long ago. Facebook the company has long stopped being an exciting new service and started seeming more like a local gas and electric utility. You never stop using its services, but you constantly complain about it.
I think it's difficult to think past one's microsociological experience to the scale of the company. The truth is that while an individual's Facebook's network growth explodes and then quiets down, the same has not been true at the global scale.
Facebook just keeps growing and growing as individual after individual experiences what you did way back when George W. Bush was still president. Look up at that graph. Facebook's really hyper growth did not take off until August of 2008, when Facebook had something like 33 million US users, 25 million of whom were between 18 and 34 years old.
Eventually, and maybe sometime in the not too-distant future, Facebook may have a hard time finding people with Internet access and socialization habits that make them easy to capture. But for now, Facebook keeps adding another 500,000 users a day, each and every day, right through every privacy debacle and user-interface snafu.
Has growth slowed a bit since 2011, when the company was adding almost 800,000 people a day? Sure. But don't mistake deceleration for contraction.
A quick note on the numbers used above. In most cases, they are the official Facebook numbers. In other cases, the numbers were quite difficult to find (700 million particularly). Here's where they all came from with estimation explanation, if necessary.
100 million: Facebook official
200 million: Facebook official
300 million: Facebook official
400 million: Facebook official
500 million: Facebook official
600 million: Goldman Sachs note
700 million: This one is tough. Facebook confirmed on July 6 that it had more than 750 million users. Socialbakers predicted Facebook would pass 700 million users in early June. But that would have meant the company adding 50 million users in a little over a month, so I picked May 15 as an earlier, somewhat arbitrary but reasonable date.
800 million: Facebook official
900 million: Facebook official
1 billion: Facebook official