An Abstract Example
At this point one may wonder how many friends a person must have in a world where every pair of individuals is at most six steps apart and everyone has about the same number of friends. First, give the world a population of a millon virtual people denoted P000000, P000001, .. P543210, .. P999998, P999999. Then imagine that Pijklmn knows everyone whose number differs by one digit — ie. P111111 knows P011111, … P911111, P101111, … P191111, P111110, … P111119 (9+9+9+9+9+9=54 others known). In this world, every Pabcdef is at most six links from any other Pijklmn, and everyone knows 54 others. To construct a model in which there are about six billion individuals, everyone would need to know 250 others [replace 0..9 by 0..42, then 43^6 is about 6billion, and individuals know about 6*(42-1)others].
-Cleaned version of a post on wiki
