Précis - On 4 October, Facebook announced that it reached one billion monthly active users on 14 September this year.

What?

In a personal message from CEO Mark Zuckerberg, Facebook announced that more than one billion people are now actively using the site on a monthly basis.  Most new users were in Brazil, India, Indonesia, Mexico and the United States. Of the one billion, 600 million are mobile users.

The announcement comes just two years and three months after Facebook hit 500 million users. Some have questioned whether the one billion figure includes fake accounts, which make up approximately 8.7% of users according to Facebook. Either way, this recent announcement makes Facebook one of the fastest growing sites in history.

So what?

The crucial question for Facebook now is how to effectively monetise its mobile subscriber base.

Following its listing in May this year, Facebook's shares fell from their initial public offering of $38 to around $21. Concerns over Facebook's ability to monetise mobile were a key factor. Interestingly, Facebook's Q3 results showed a huge spike in mobile advertising revenue from just $10 million in Q2 to $150 million in Q3.

Speaking at a technology conference in September, Zuckerberg admitted that the performance of Facebook stock had been disappointing. Zuckerberg also stated that a key factor in determining Facebook's performance, at least in the next 3 to 5 years, would be how well Facebook is able to generate revenue from mobile usage.

One billion users is an impressive achievement. However, investors will be primarily concerned with how Facebook plans to turn this figure into profit. With 600 million users now on mobile devices and the trend towards mobile usage expected to continue, it remains to be seen how successfully Facebook can grow its impressive Q3 results from mobile advertising.