SOLVED! Found the cause of the disappearing FB button. JFBCLogin module is conflicting with the modified Activity Stream Module from JomSocial. The problem occurs only when the activity stream module had been modified by Bernard's hack to enable video embedding in the activity stream. Well, this feature is too nice to let go, so the only best way is not to put both modules in the same page.
Unless, anybody else has a better suggestion ..... pls share ...
Thanks.