Who decides societal norms and what is the criteria for selecting/passing on those norms?

Yesterday, I participated in a Fluther question that stirred a strong reaction in me. At first I thought my reaction was the result of conflict and the need to defend/prove my college-level studies in social sciences and multi-cultural education.

Later I realized something much more simple. The question tapped at my sentimental attachment to my all-American childhood.  Having been raised in a small town, I have fond memories of various local events and experiences. I remember Arbor Day, watching neighbors pass around and plant tree seedlings.  I remember the Halloween parade and riding on the float dressed as a ghost along with other costume-wearing members of my Girl Scout troup. I remember Christmas and receiving a gift delivered by a man dressed as Santa escorted by the local Sheriff.  I remember Faschnaut Day and other quirky events observed in my hometown.

I cherish these memories, and I thought how sad it is for another child not to experience similar memory-producing events. It seems normal to me for local society to celebrate certain events, experiences, beliefs, and practices. It seems abnormal to me not to be a part of societal happenings.  Yet, I realize that my experiences are personal to me and may not be of value or interest to someone else.  Therefore, I’m not suggesting my experiences are the standard for critiquing others.

Maybe for some, societal norms have nothing to do with holidays or events, but with manners, values, and etiquette.

The question is not about the importance of societal norms.  Based on the discussion in the other Fluther question, it seems agreed that societal norms are important to have and to pass on to subsequent generations.

The question here is about the who and the how.  In the 21st Century, in the age of the Internet and the globalization of just about everything, who in society decides the norms for all to practice? What is the criteria for deciding the norms worth practicing?