The idea that any two people on Facebook can be connected through no more than 4 mutual friendships.
The average person has 130 friends on Facebook, meaning that they have on average 130² friends of friends, 130³ friends of friends of friends, and so on. In six degrees that encompasses 130^5 people. This is 3.7 billion people, more than the entire global Facebook community.
Jane is friends with Mark
who is friends with Karisa
who is friends with Oleshia
who is friends with Chenwei
who is friends with 손재호
Six Degrees of Facebook Separation