Facebook on Thursday celebrated eclipsing the billion-member mark, touting its mission to make the world more social while investors wondered how the service would cash in on its popularity.

Co-founder and chief executive Mark Zuckerberg announced that more than a seventh of the planet's population resided virtually at Facebook, saying the accomplishment was "humbling."

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

Estranged Facebook co-founder Eduardo Saverin weighed in on his feed at the social network, writing "Congrats to Mark and the entire Facebook team and especially to every Facebook user who built Facebook with every login, invitation, comment and feedback."

Facebook even released its first promotional video in tribute to the milestone.

Investors, however, did not appear celebratory and Facebook shares slid about 1.6 per cent to $21.60 in after-market trading on the Nasdaq.

Analysts are divided on whether Facebook can leverage its massive user base for advertising and other revenues and still remain loyal to Zuckerberg's goal of making it a service to connect the world.

Of particular concern was how Facebook planned to make money from members shifting to accessing the service on smartphones or tablet computers.

More than half of the billion people using Facebook monthly do so on mobile devices, according to the northern California-based company.

Trip Chowdhry at Global Equities Research remained bearish on Facebook, saying the company has failed to come up with a credible expansion plan.

"OK, one billion users, now what? How do you monetize them?" Chowdhry said. "The problems with Facebook remain the same."

Facebook revenue this year "will not fly as high" as earlier estimated, according to industry tracker eMarketer.

The company should claim a 14.4 per cent share of the $14.98 billion US online display ad market while Google takes a 15.4 per cent share, eMarketer forecast.

Only $72.7 million of the social network's revenue will come from ads served on mobile devices such as smartphones while Google is expected to reap $1.4 billion in that arena, according to eMarketer.

Mobile advertising is a "long-term play" for Facebook with multiplying in the coming two years, eMarketer predicted.
Lou Kerner of the Social Internet Fund said the market is underestimating Facebook's ability to drive revenues higher.

"Engagement per user continues to grow rapidly, and that's the metric that will increasingly drive revenue," he said.
Facebook said it reached one billion monthly active users on September 14.

VietNamNet/AFP