After reading a few blogs I've worked out that when Americans say 'room mate' they really mean 'flat mate.' I think. I hope so, anyway. I haven't shared a room with someone on a semi-permanent basis since I was ten and I hate to think the housing crisis is so severe in the USA that a large proportion of generation-X has to share a room, like some episode of the Brady Bunch.
Come to think of it, the house in that show was huge. The guy was a bloody architect and the house had a den, a study, a lobby, a sunken living room, a dining room, a master bedroom with walk in wardrobe and en-suite but six adolescents had to share two bedrooms. And then one of them gets the chance to move into the attic but prefers to move back into the bedroom with his two dopey brothers. Triple bunks for Chrissakes, TRIPLE BUNKS!!!