For explaining the usefulness of social media to Digital Immigrants I can think of no better tool than examples of what other communities and agencies are doing. They may poo poo the idea in the board room, but they also don't want to be left behind or look backwards. The mindset is that of settlers versus pioneers. Pioneers are the ones who had arrows in their back and they don't want to end up that way. But, they may be willing to be settlers in social media and build on the success of the pioneers. Or, they may want to stay in the old country and wag their heads at anyone wanting to make the digital journey.
To help in the endeavor of providing those examples you need to check out Citizen 2.0 which has 17 examples of social media and government innovation. Some of these you may have heard of before, but there are others I'm sure you have not read about.
Print out a nice color copy of the document and attach a note and put it in a decision makers "In Box" should such a physical thing still exist.