If you have previously recommended this thread, you should see a tick / check mark on the recommend button. Click the tick to undo the recommendation (the tick may appear to change to a cross as you do this.) If you added a comment with your recommendation, you will need to delete that from your facebook wall separately.
It would appear that hidden threads are listed by date of thread commencement, which can make them hard to find if you hide a long running thread, as I often do by accident due to big fingers and tiny phone screen.
Would it be possible to have them list by date of being hidden? Or even by date of last post?