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.
I might as well post a few things whilst I'm here!!
1. If a thread title is particularly long, I can't see the whole thing, it cuts off the end.
2. When I go into a thread, then come out, the title is stuck at the top of the page. Event when I go into another thread, the old thread title still shows. I have to turn my phone off to get rid of it.
3. I tried to post this from my phone, but I can't post from it at all. The + button doesn;t work at all.