Think of it this way. Let’s say the government suspects you are a terrorist and it has access to your Facebook account. If you’re an American citizen, it can’t do that currently (with certain exceptions)—but for the sake of argument. So all of your friends, that’s one hop. Your friends’ friends, whether you know them or not—two hops. Your friends’ friends’ friends, whoever they happen to be, are that third hop. That’s a massive group of people that the NSA apparently considers fair game.
For a sense of scale, researchers at the University of Milan found in 2011 that everyone on the Internet was, on average, 4.74 steps away from anyone else. The NSA explores relationships up to three of those steps. (See our conversation with the ACLU’s Alex Abdo on this.)