We've seen issues on using Add This with Facebook. Logged in to Facebook I can get a page to appear on my timeline when I like it, but if I'm logged out I can't get it to appear - i.e., once I authenticate it does nothing.
My client who has also tested this can't get it to work either logged in or logged out. Our QA company verified the issue with logged out being problematic and said they had seen this behaviour on other sites.
We've run your internal code check, which says our code is fine. An example page is below.