theorem of facebook circularity

If someone finds a random person on Facebook and clicks on a random friend, then clicks on that random person's random friend and so on and so forth, then the original person (the clicker) will eventually circle back until they find someone with a "mutual friend."

Bob sees Sally's profile. In Sally's profile, Bob click's on Terry's friend icon. After being rerouted to Terry's profile, Bob clicks on Jerry's friend icon...etc...until Bob finds a mutual friend. This is due to the "theorem of facebook circularity."