In an interview published Thursday, Mark Zuckerberg revealed that Facebook registered its 1 billionth user on Sept. 14, eight years after the social-networking site launched at Harvard. “It’s really humbling to get a billion people to do anything,” Zuckerberg told Businessweek. The Facebook team came together in the company’s offices to watch the countdown, but did not publicly announce the milestone because of the difficulty of pinpointing the billionth user. Zuckerberg said Facebook has just 1,000 engineers, which means each is responsible for a million users. The social network also released its first commercial on Thursday, which attempts to link Facebook to the human desire to connect.