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 don't know if this is MN or my computer (being a luddite).
I keep getting logged out. Just now I've tried several times to comment on an AIBU thread, and each time I click on it, it logs me out. I log back in, and it logs me out as soon as I click on the thread.