>I, the user, go to Facebook, expecting to see the widget there, but it's not there. I'll just assume it's lag. Things are slow sometimes. But it never shows up

>I, the user, go to Facebook, expecting to see the widget there, but it's not there. I'll just assume it's lag. Things are slow sometimes. But it never shows up. I'm left with the assumption that it's Foobar's fault, for having a buggy "Share to Facebook" button. If the button doesn't give you error feedback when you click it then it is buggy. The author has to assume anything an everything will give an error at some point, and reporting errors to users is important. Saying "everything is great!" and then erroring silently in the background is lazy and deserving of a user's ire.