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.
How does it work?? If I click the Facebook button does it show up on just my news feed or does it show on the Mumsnet page so anyone who likes that page can see it?
It'll show in your feed, EverythingIsTicketyBoo. It's a two-step process, so you can have a go and see what it would look like - and then click away if you decide you don't want to post it onto your feed after all.