I've never used it but my guess is it became mostly a casual sex app based on the mechanics of the interface. A picture of the potential date appears and the user can swype left or right to accept or not. I think the viewer can view more pictures or a text profile but many probably pick the person because of the first picture they see. So the app excels at letting the user make fast decisions based on physical attraction which appeals to many that just want sex. It's efficient for them to use. I thought that was obvious but maybe not many people see it that way?