Facebook announced that it crossed a billion users in mid-September.

Mark Zuckerberg handled the announcement that his company is serving one-seventh of the world's population exactly as we recommended—don't make a big deal, and say that the company's focused on the next billion users.

Still, it's a milestone, and Zuckerberg did an interview with Bloomberg Businessweek's Ashlee Vance to mark the occasion.

Vance asked Zuckerberg if Facebook did anything special for User No. 1,000,000,000, "grocery-store style."

The short answer: Zuckerberg doesn't know who that person is.

The long answer:

I wanted to, but we didn’t want to leak that we got to a billion people. Doing data analytics at this scale is a big challenge, and one of the things you have to do is sample. So it’s funny, we were all sitting around watching us get to a billion users but it was actually just a sample of the users. It’s like you’re not going to try to pull a billion rows from a database, so you’ll pull a sample and project out. I don’t even know if we knew who the billionth person was.

In other words, Facebook had to estimate its user count because actually counting them user by user would have been too taxing on its servers. So User No. 1,000,000,000's identity will remain a mystery.