Imagine a person with 100 friends. If each of his friends also has 100 friends, then the number of friends-of-friends will be 10,000. If each of those friends-of-friends also has 100 friends then the number of friends-of-friends-of-friends will be 1,000,000. Some of those friends may overlap, so we need to filter down to the unique connections. We’re only two hops away and the number is already big. In reality this number grows even faster since most people on Facebook have more than 100 friends. We also need to do this computation 1.6 billion times; that is, for every person on Facebook.
You can visit there to see where you fall in the spectrum. While most people have 100 FB friends, most people also don’t know all of their friends in real life. Facebook is a virtual world so you might collect more friends than you actually know. I had 1500 friends at one point. I didn’t know all of them. I think people like me skew that figure to be much lower than the well-documented 6-degrees of separation. FB says it’s more like 3 degrees.