In short, the gist of Clay Shirky's speech is this...
In the industrial revolution, people were drawn into cities; productivity created a boom in free time (vs. the old agrarian lifestyle) and the "opiate of the people" (to steal a concept from another historical figure) was GIN (or at least it was for London, which Clay talks about). The boom in free time (and time to think about things) is what Clay calls a "Cognitive Surplus" and in the industrial revolution it was eventually deployed by society in the form of libraries, schools and other institutions which helped society.
He goes on to point out that, [in the post world-war growth of mass media and business growth], productivity of our business systems has continued to grow and the replacement for GIN is television. (specificically, according to Clay, TV sitcoms).
His question is ... how are we, as a society, going to use our growing "Cognitive Surplus" (instead of just watching more sitcoms on TV)?
Clearly, Toastmasters is a great choice and the good news is that you don't have to give up sitcoms "cold turkey". There are now DVR's which will tape your shows and play them back (without commercials) so you can attend your evening Toastmasters meetings !
What are you going to do with your Cognitive Surplus ?