I understand that - that's exactly what I'm pointing out.
Stripping HTML is fine, but you should replace blocks surrounded in <p> with two newlines, or instances of <br /> with one newline or the formatting is completely wrong.
Just like going in the opposite direction, a nl2br is a good idea, you should be doing the opposite when preparing the text to go to Facebook.
Since this code can't be overridden, I have to suggest that this is a bug.