facebook_logout()
Returns: N/A
With this function you can log the user out of Facebook. Note,
this does not mean that Facebook is disconnected from the game and
there is no need to call facebook_init again if you wish
the user to log back in again. It is also worth noting that certain
graph and dialogue functions may work, only that they will prompt
the user to log in again beforehand.
facebook_logout();
The above code simply logs the current user out of Facebook.