Students have been using laptops in my classes for more than a dozen years. They were never using them for the sole purpose of taking notes, especially since the installation of WiFi. Smart phones don't change anything.
If you teach a class that engages students and gets them to debate and discuss the issues then they won't have time to live tweet or do anything else on their smart phones. If you are teaching difficult and challenging concepts and principles, then you are likely to keep their attention. If you teach a class that allows students to busy themselves with lots of other tasks during the lecture, and still get a good grade, then they will do that.
I pose the following question ... if your students can spend a considerable amount of time using their smart phones during class then they are not giving the class their full attention. Are you teaching effectively?
And whatever happened to note-taking? How can you take notes with a smart phone in your hand? Is taking notes an old fashioned concept that should be abandoned?