Facebook's announcement that it has hit 1 billion users has generated a media storm. The number — though massive and round, a beacon of Facebook's immense scale — isn't really news.
We've known Facebook has been steadily heading here over the past six months. Far more interesting is the fact that Facebook reported it has 600 million mobile users. (Read More:Will Facebook’s New Focus on Revenue Boost Its Stock?)
Why is Facebook's smaller mobile number more important?
Because it's growing faster than Facebook's overall user base, and much faster than the number of desktop users.
Bottom line — nearly all Facebook's growth is coming from mobile. This is a great opportunity, and a precarious risk.
On one hand, Facebook has a chance of targeting the 5 billion people with phones, a much larger potential audience than the number of people with computers.
But Facebook is still burdened by the fact that it isn't making as much money from mobile users as it is from users of its desktop offerings. Until mobile shifts into becoming as profitable — or more profitable — than the desktop, Wall Street will continue to be worried. (Read More: What Caught Wall Street's Eye in Facebook’s New Ad.)
Mobile advertising has great potential — total spending is projected to grow 80 percent this year according to eMarketer. But it's coming off a very small base — eMarketer projects somewhere in the ballpark of just $70 million in mobile ads for Facebook this year.
But its not just advertising. Facebook's recent comments about other potential revenue streams — e-commerce and search — could also capitalize on those mobile users. It's worth noting that Facebook's Gifts tool was built as a mobile app. (Read More: Facebook Moves Into E-Commerce, Tests Facebook Gifts.)
Facebook revealed some other interesting data points about its users in its 1 billion announcement.
Despite the impression that Facebook is losing its cool factor because everyone's grandmother has joined, the median age has actually dropped. Facebook users' median age now is "about 22" — when the social network hit 500 million in July 2010 the median age was "about 23."
—By CNBC's Julia Boorstin; Follow Her on Twitter @JBoorstin
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