On the afternoon of Nov. 16, 2010, Mark Zuckerberg was leading a meeting in the Aquarium, one of Facebook's conference rooms, so named because it's in the middle of a huge work space and has glass walls on three sides so everybody can see in. Conference rooms are a big deal at Facebook because they're the only places anybody has any privacy at all, even the bare minimum of privacy the Aquarium gets you. Otherwise the space is open plan: no cubicles, no offices, no walls, just a rolling tundra of office furniture. Sheryl Sandberg, Facebook's COO, who used to be Lawrence Summers' chief of staff at the Treasury Department, doesn't have an office. Zuckerberg, Facebook's CEO and co-founder and presiding visionary, doesn't have an office.

The team was going over the launch of Facebook's revamped Messages service, which had happened the day before and gone off without a hitch or rather without more than the usual number of hitches. Zuckerberg kept the meeting on track, pushing briskly through his points - no notes or whiteboard, just talking with his hands - but the tone was relaxed. Much has been made of Zuckerberg's legendarily awkward social manner, but in a room like this, he's the Silicon Valley equivalent of George Plimpton. He bantered with Andrew "Boz" Bosworth, a director of engineering who ran the project. (Boz was Zuckerberg's instructor in a course on artificial intelligence when they were at Harvard. He says his future boss didn't do very well. Though, in fairness, Zuckerberg did invent Facebook that semester.) Apart from a journalist sitting in the corner, no one in the room looked over 30, and apart from the journalist's public relations escort, it was boys only. 

The door opened, and a distinguished-looking gray-haired man burst in - it's the only way to describe his entrance - trailed by a couple of deputies. He was both the oldest person in the room by 20 years and the only one wearing a suit. He was in the building, he explained with the delighted air of a man about to secure ironclad bragging rights forever, and he just had to stop in and introduce himself to Zuckerberg: Robert Mueller, director of the FBI, pleased to meet you.

They shook hands and chatted about nothing for a couple of minutes, and then Mueller left. There was a giddy silence while everybody just looked at one another as if to say, What the hell just happened?

It's a fair question. Almost seven years ago, in February 2004, when Zuckerberg was a 19-year-old sophomore at Harvard, he started a Web service from his dorm. It was called Thefacebook.com, and it was billed as "an online directory that connects people through social networks at colleges." This year, Facebook - now minus the the - added its 550 millionth member. One out of every dozen people on the planet has a Facebook account. They speak 75 languages and collectively lavish more than 700 billion minutes on Facebook every month. Last month the site accounted for 1 out of 4 American page views. Its membership is currently growing at a rate of about 700,000 people a day. 

What just happened? In less than seven years, Zuckerberg wired together a twelfth of humanity into a single network, thereby creating a social entity almost twice as large as the U.S. If Facebook were a country it would be the third largest, behind only China and India. It started out as a lark, a diversion, but it has turned into something real, something that has changed the way human beings relate to one another on a species-wide scale. We are now running our social lives through a for-profit network that, on paper at least, has made Zuckerberg a billionaire six times over.

Facebook has merged with the social fabric of American life, and not just American but human life: nearly half of all Americans have a Facebook account, but 70% of Facebook users live outside the U.S. It's a permanent fact of our global social reality. We have entered the Facebook age, and Mark Zuckerberg is the man who brought us here.

Zuckerberg is part of the last generation of human beings who will remember life before the Internet, though only just. He was born in 1984 and grew up in Dobbs Ferry, N.Y., the son of a dentist - Painless Dr. Z's slogan was, and is, "We cater to cowards." Mark has three sisters, the eldest of whom, Randi, is now Facebook's head of consumer marketing and social-good initiatives. It was a supportive household that produced confident children. The young Mark was "strong-willed and relentless," according to his father Ed. "For some kids, their questions could be answered with a simple yes or no," he says. "For Mark, if he asked for something, yes by itself would work, but no required much more. If you were going to say no to him, you had better be prepared with a strong argument backed by facts, experiences, logic, reasons. We envisioned him becoming a lawyer one day, with a near 100% success rate of convincing juries."