SAN FRANCISCO — Facebook has hit the biggest milestone in the company’s eight-year history: 1 billion users.

Company founder and Chief Executive Mark Zuckerberg made it official with a post on his Facebook Timeline on Thursday morning: The social networking giant has wired one-seventh of the world’s population.

“Helping a billion people connect is amazing, humbling and by far the thing I am most proud of in my life,” Zuckerberg wrote.

Internally, Facebook employees followed the live countdown and quietly popped the champagne corks on Sept. 14 when the company hit 1 billion users. The achievement is particularly meaningful to Zuckerberg, who started the company in his Harvard dormitory room and has never been shy about his ambition of connecting everyone on the planet.

In June 2010, Zuckerberg said it was “almost a guarantee” that Facebook would reach 1 billion users. As of the end of June 2012, the company reported it had more than 955 million monthly active users. In February, when it filed for its initial public stock offering, it said it had 845 million users. Facebook crossed over the 500-million user threshold in 2010. The hit film about Facebook, “The Social Network,” was promoted with the phrase: “You don’t get to 500 million friends without making a few enemies.” The statistic was out of date before the movie even opened.

Facebook held an on-campus hackathon Wednesday night to commemorate the milestone of 1 billion monthly active users. Facebook defines active users as people who have logged on to Facebook within the past 30 days. And getting users more engaged is one of the challenges the company faces.

“Now we have to make them 1 billion daily active users,” engineering manager Pedram Keyani exhorted a crowd of hundreds of employees who had gathered to kick off the hackathon.

As the announcement was made, Zuckerberg was on a flight home from Russia, where he looked to boost Facebook in that country by meeting with Prime Minister Dmitry Medvedev, appearing on a popular late-night talk show and judging a competition for Russian programmers. He’s expected back at Facebook’s Menlo Park, Calif., campus Thursday for a festive all-hands meeting and a party.

Facebook apparently won’t be resting on its laurels. It must prove its worth to investors, not just users, and is already focused on the next billion users, and in getting them more engaged with the service. The theme of Wednesday’s hackathon was the next billion.

That task has taken on more urgency as Facebook’s rocketing growth has cooled. The company is looking to reach new users on mobile devices. Some of Facebook’s fastest-growing markets are in developing countries, where most people use mobile devices to communicate.

The next billion will be a lot tougher than the first. One big reason: A significant chunk of the world’s population can’t access Facebook at all. The Chinese government has blocked access to the website since 2009, although many still scale the “great firewall” to use it.

It’s a big gap for Facebook. The Chinese are avid users of social networks. Zuckerberg has said that Facebook has no immediate plans to enter China.

“There are so many other places in the world where we can connect more people more easily,” he told interviewer Charlie Rose in February.