The Moon who Fell to Earth?

So, if the Moon really is formed from the conglomeration of debris created from the collision of another planet with Earth billions of years ago, how is it that as the particles combined, and the mass increased as a result, the orbit of the forming Moon didn't reduce increasingly to the point where it fell to Earth? How can an object of increasing mass orbiting the Earth settle into a static orbit?

via JREF Forum http://ift.tt/1pVuRcC