'On hi-score' triggers once for each score in the score board after a 'request hi-score board' action.
E.g. if you have 5 top scores, 'on hi-score' triggers 5 times. You have to append to a text object's text (or some other progressive way), otherwise if you just set the text object's text you'll overwrite each score and end up only showing one score.
Have you sumbitted a non-zero score before showing the hi-score board? It won't count as submitted until the 'on score submitted' trigger runs, so if you request hi-scores at the same time you submit a score your new score probably won't show up. Request the hi-score board in 'on score submitted' instead.
Also note only you and your friend's hi-scores show up - that's how facebook works. You won't see scores for anyone who's not friends with you on facebook.