One of the threats to Facebook I see is its user base getting wise on how to game the platform, making the data and patterns it collects essentially unreliable for marketing and advertising purposes. The simplest of ways would be to set up multiple FB accounts as there is no true identity verfication, so that your activity becomes dispersed.
Facebook is its own biggest threat and always has been. At some point fear, greed and control-freakness drove them to the top but these are the same exact qualities that will bring them to the bottom