Pretty easy to be immune to spam when most of the interactions are between known people. I don't know why FB and other sites doesn't adopt a 'secret handshake'

Pretty easy to be immune to spam when most of the interactions are between known people. I don't know why FB and other sites doesn't adopt a 'secret handshake' code for friend requests. It would eliminate all of the unwanted requests if someone only accepts the secret handshake.