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 can view threads quite happily but if I try and and look at Threads I'm Watching for example it says that I need to log in.
However when I put in my details and click the 'Log In' box it goes back to the page with the all the Active/ I'm On/ I'm watching/Customize etc boxes and the 'you need to log in to see this page' message again
Potentially a privacy issue as it lets websites track you. However it is needed for many things, shopping carts for example. So in my view better to have it than not. Cookies are stored locally on your device and only readable by the website that created the cookie.