Facebook tops 1.5 billion monthly users

Facebook crossed the 1 billion monthly user mark in September 2012, so it's taken about three years to add the last half billion. It took just over two years to amass the half billion before that. For comparison, Twitter currently has about 320 million monthly users.

Facebook announced the figure with its earnings results for the third quarter, which came in better than expected. Revenue was $4.5 billion, up 41 percent from a year earlier, the company said, while net profit was 896 million, up 11 percent.

Excluding charges, Facebook's profit was $0.57 a share, better than the $0.52 analysts were expecting, according to a poll by Thompson Reuters.

Facebook had 1.55 billion monthly active users at the end of September, up 14 percent from the same time last year. Mobile monthly active users were 1.39 billion, it said.

The social network announced another milestone in August, when it said the number of people who accessed Facebook in a single day passed 1 billion for the first time. 

"When we talk about our financials, we use average numbers, but this is different. This was the first time we reached this milestone, and it's just the beginning of connecting the whole world," Zuckerberg said at the time.

The company's net profit had declined in the two previous quarters, largely because costs increased as it invested in new areas like virtual reality. But costs rose less sharply this past quarter -- by 68 percent, compared to 82 percent in the second quarter -- and net profit was up again.