Facebook Hits 2 Billion-User Mark, Doubling in Size Since 2012 - The New York Times

Also see Facebook’s First, Second, and Third Billions: How Facebook got here, and how it’ll get there (Beyond Devices)

"The user base is bigger than the population of any single country, and of six of the seven continents. It represents more than a quarter of the world's 7.5 billion people.

Facebook defines a monthly active user as a registered Facebook user who logged in and visited Facebook through its website or a mobile device, or used its Messenger app, in the past 30 days. It does not include people who use the Instagram or WhatsApp networks but not Facebook."

Facebook Hits 2 Billion-User Mark, Doubling in Size Since 2012 - The New York Times