It is difficult to guess what is causing it, but my first suspicion would be mechanical issue with mouse or keyboard. If you can, try changing those and see, if it disappears. Next would be checking various accessibility options in preferences - may be some "sticky" option is selected and it causes system to think you are holding down a specific button. New clean user account on that computer would help distinguish since it would have default preferences but same hardware. If none of those help, you will need to try to figure out what is provoking it and that might help you identify culprit.
You know, Facebook is practically virus - for our computers and minds and souls, combined. Not sure how are you accessing it, but if it happens mostly (only) when using FB page, I'd suspect some issues between your browser setup and FB page programming. In that case I suggest ignoring FB and your life (and computer) will improve ;-)
