I mentioned NPR because that is what I happened to be listening to at the time, but they seem to prefer Twitter over Facebook (vs. BBC, which seems to be the opposite). NPR's physical corporate address is on their site, and if you were to send a physical sign to the organization with the noted personality addressed properly, I believe it would easily get to its destination. Put your business card in the same package.

Same for any other organization: just send these signs to the physical addresses, and consider the costs your advertising/ promotions costs. I'm sure some "social media consultants" would love to have @twitterusername signs on their office doors to demonstrate to their clients how savvy they are.