* Roman Catholics (1.2 billion)
* People living in China (1.4 billion)
* Total speakers of English, the most widely-spoken language in the world (1.4 billion)
* Muslims (1.8 billion)
* Viewers of the 2014 FIFA World Cup final (1 billion)
* iPhones ever manufactured (1.2 billion; also, the top two most-used apps are Facebook and Facebook Messenger)
* Windows users (1.4 billion)
* Inhabitants of the European Union (508 million)
* Cats (600 million)
* Firearms (875 million)
* Africans (1 billion)
Now, obviously whether or not you use Facebook isn't as important to your identity or as hard to change as your religion or where you live, but it's also not something that's just going to suddenly disappear without some sort of replacement, or else if it does, it will be the first time in human history that two billion people all decided at once to give up on something.