I think the facebook button that you've added at the top is interfering with the built-in one. I see these 2 warnings in the js console:
Code:
The "fb-root" div has not been created, auto-creating
FB.getLoginStatus() called before calling FB.init().
I think it might be because the new buttons call the facebook script before the html for the built-in vb button is reached. I'm not sure what to do about that - maybe try moving the js for the add buttons to be later in the code? I guess if it's a mod that will be difficult.