Hardware-level user tracking? You just know they must be thinking about doing this.
Sorry you are getting down-voted. In fact, Intel and AMD have thousands of undocumented instructions that folks have only started [1] to enumerate. Even then, some of the instructions may not be visible through this method of enumeration. FB would have the option to add their own instructions. I would be curious to see how transparent they might be around the production and documentation of these chips.