A symbiosis has developed between the family, Greta/Fox and obsessive viewers. The family feeds information based on rumor and speculation to keep their daughter's name in the news, the media promotes these stories because it gives them something new to talk about and hence maintain their ratings and the obsessive viewers keep watching because, well they are obsessed.
You are ready to call millions of viewers "obsessed" because of their interest in a drama like this? Instead of passing it off to obsession, why not do a little analysis. If we grant the fact that most TV "news" is actually entertainment, then what's wrong with being entertained? Reality shows are all the rage on TV (or were, I'm not sure), so there isn't much of a mystery as to why Greta is getting the ratings.