Mark Zuckerberg has just six billion more people to go. Facebook said it has topped one billion monthly active users, a milestone that raises growth questions anew for the social network and whether it can profit off its giant user base. Geoffrey Fowler reports on digits. Photo: Getty Images.

Mr. Zuckerberg said Thursday that Facebook Inc., FB +0.54% the social network he started in his Harvard dorm eight years ago, reached the milestone of one billion monthly active members on Sept. 14.

The Menlo Park, Calif., company had reached 500 million active users in July 2010, making Facebook one of the fastest-growing sites in history.

Facebook didn't specify whether the one-billion count includes fake accounts, but it has been on a campaign to remove them. In August, Facebook said fakes constituted 8.7% of its users.

Facebook founder and CEO Mark Zuckerberg in San Francisco on Sept. 11.

The announcement couldn't come soon enough for Facebook. Investors today value the business at a fraction of the $100 billion opportunity they saw in the company when it held its initial public offering in May.

Other social media businesses are also running into difficulties. On Thursday, social-games maker Zynga Inc. lowered its earnings outlook for the year, in part because consumers appear to be losing interest in some of its games.

The question for Facebook is just how much value Mr. Zuckerberg can extract from all those people that Facebook signed up for free. Last year, the company produced nearly $4 billion in revenue, mostly from advertising—far less than the nearly $38 billion that Google squeezed out of an equally large set of users in the last fiscal year.

Facebook still trails behind other Web companies in capturing revenue from users. In the U.S., search engines such as Google and Yahoo make about $88 per person who uses its search engine, while Facebook makes about $15 per user, according to eMarketer Inc.

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What of other books with different prefix.. Zuckerberg making money with my-Likes, my-comments, my-pictures and even... my-name just like that