for me facebook highscores work perfectly, but i'm managing them through php, not with facebook plugin. keep in ming, that scores don't appear anywhere on facebook until some of your friends beat them. so if you get high score of, say, 100, it will not appear on any walls. only when your friend get score of, say, 120, it will appear that he/she beat you in your game with said scores.
also, my latest child is prepared to be multiexport (here linked to facebook, but also available on scirra arcade and soon on kongregate) - and uses facebook scores and achievements, even when on arcade on kongregate, the only requirement is to connect the game on facebook once.
feel free to ask anything, but you have to have intermediate knowledge of php and facebook api to make this stuff work.