The stones fall apart eventually, but I reckon you could use them a few times before they completely fell asunder. Some of the fulachts were obviously used for centuries, because the mounds are several metres high and the same in width (Deerpark, Ballymacarbry, Waterford inventory). Its possible that the 'exhausted' stones were subsequently used for trunking for tracks, foundations etc so that there was'nt an inordinate amount of stuff around the site.