And maybe the American fans of the show have been progressively dwindling,too.
you betcha it's been dwindling for some time now. a 6.9 ratings share is excessively low when you compare it to the pilot episode, which garnered 14.10, equivalent to 14 million viewers. season 1 of
Heroes maintained an average of 13-14 million viewers throughout, while season 2 decreased to 10-11 million viewers. season 3 currently averages around 7-8 million viewers, so Heroes has lost almost half of its loyal viewer base since season 1.
as far as how exactly the ratings system works, i'm not sure--Nielsen Media Research handles the ratings measurements in the USA (and i think, even here). they use devices called set meters or people meters that are installed in selected homes to record the viewing habits of those homes. recently, they've added data from the usage of digital video recording devices like TiVo as well, so now even delayed and recorded viewing of a TV show will count as a rating point for it.