NEW YORK (AP) -- More than a billion people now log into Facebook each month to check up on old friends, tag photos of new ones and post about politics, religion, cats or what their kids are doing.

CEO Mark Zuckerberg marked the milestone on his Facebook page, as he has in the past when the site's users hit nice round numbers.

That's double the 500 million it hit in July 2010 -- what now seems like a lifetime but was a little more than two years ago. August 2008 marked another milestone, 100 million.

The latest milestone also amounts to nearly half of the world's roughly 2.5 billion Internet users, as measured by the International Telecommunications Union.

Most of the people, some 81 percent, live outside of the U.S. and Canada. Many of them log in on mobile devices rather than personal computers, and the company now has 600 million mobile users.

The people joining now are young, with a median age of 22. It was 23 in 2010 and 26 in 2008 and 2007. Most of them are from Brazil, India, Indonesia, Mexico and the United States. They are unlikely to be from China, the world's most populous country and home to its largest Internet population. And millions of them are not actual people. Facebook acknowledged in August that 8.7 percent of its then-955 million users may be duplicate or false accounts. At that rate, as many as 87 million accounts are fake.

As expected, the longer users are on Facebook, the more "friends" they have on the site. A user who signed up two years ago has an average of 305 friends. Someone who signed up in January 2006, when Facebook had 25 million users, now has nearly 600 friends, on average.