I've always been under the notion that anything mechanical/technological can't be random. There's always a method to the madness.
It's obviously not something very prominent to the average user, if it were it could be used as a tool of harassment. E.g. Person A visiting person B's page countless amounts of times, thus appearing on the top of their friend's list. Could be interpreted as stalking.
If someone comes up with a better interpretation, please do dazzle me. I just can't comprehend "randomness" as a correct answer.