hype.- A fad. A clever marketing strategy which a product is advertized as the thing everyone must have, to the point where people begin to feel they need to consume it. (or in this case read it)
The "hype" around 50 Shades was the S & M aspect, it generated a lot of discussion, with people then reading the book to see what all the fuss was about, then they said 'have you read it because ...' and then more people read it to see what the fuss was about. Once the advertising people start the ball rolling it becomes self perpetuating - see here we are discussing it.
If every one shrugged and said 'so what', or focused on the actual quality of the prose etc I'm sure it would have died a natural death rather rapidly.