This article is based on self-reporting by teenagers. How can it be used for concluding, well, anything? If you want to see how things like pregnancies are decreasing among teenagers, you would have to get that data from hospitals or such.

The only thing this tells me is that teenagers have become less comfortable about revealing anything about their sex life or smoking. In an age where Facebook photos and Tweets can cause you to lose your job, that is entirely unsurprising.